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Community and Q&A

Sealing a Concrete Thermal Water Storage Tank

mhenson | Posted in Energy Efficiency and Durability on

I am building a new off-grid house in the mountains of Colorado this coming spring.  Climate is about 4 though it rarely gets below -5F. The electrical solar system is already in place (16KW) Canadian Solar 430 watt panels.  The current plan is to insulate with R40 double stud walls and R60 roof.  Hydronic heated pex tube floor with 4-6 inches of rigid foam under the slab. It is in a good sun area, but the house will not be passive designed.  I don’t want to burn propane.  My current plan is to use 17 30-tube evacuated solar collectors or a second electric array, to heat a 5-10,000 gallon insulated, buried, thermal storage tank.  I want to keep the water at a high of around 180 degrees.  All surplus power from the existing electric array will be relayed to a heating element in the thermal storage as well (if it isn’t already up to temp).  The current problem is to build the thermal storage tank AND be cost effective.  I can’t fit a $30K+ fiberglass thermal tank into the budget.  Because of thermal cycling many have said concrete tanks will crack over time.  I can’t find any EPDM etc. liners that are rated for 200F+ degree that don’t also say intermittent and that the manufacturers won’t guarantee for more than 5 years at 180 degree temps. If I do a concrete tank and then put 1/8 to 1/4 inch of RTV high temp silicone (troweled onto the entire inside) would that hold up and/or if any cracks do get big enough to separate the silicone can I drain the tank and fill the cracks with more of the same.  The engineering numbers seem to work with everything else, but I can’t find a cost effective thermal storage tank solution as of yet. Yes, I have looked at sand and a few other solutions, but have determined a water filled tank buried just outside the house will be the most effective solution whether it is heated by a second electric solar array or evacuated tube panels.  I am open to other solutions, one guy even told me if I heat the floor during the day I should be good, but he didn’t have any numbers to prove it.  As a backup heat system I am either going to run an Arctic air-to-water heat pump or at worst a high efficiency on demand propane boiler.  Any ideas or help with thermal storage will be greatly appreciated.  Well any ideas on any of it are welcome!

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  1. Expert Member
    BILL WICHERS | | #1

    You may want to consider spray-on bed liner, the stuff used on pickup truck beds, instead of troweled RTV here. I don’t about the temperature issue with this material, but I’ve seen it used in industrial facilities before (if you drive a Chrysler vehicle, your transmission’s gears went down a chute covered with bed liner at the heat treating plant, for example). Might be worth looking into.


  2. Expert Member
    NICK KEENAN | | #2

    Two questions here: First, is high-temp RTV suitable for this application? Probably not. Continuous immersion is a really demanding environment, not one that RTV is generally rated for. High temperature is also a highly demanding environment. I don't know off-hand of any fluid-applied product that is rated for both.

    Second question though is, Is this a good idea? The last time I priced evacuated tube solar collectors they weren't cost competitive or space competitive with the combination of PV and a heat pump. Since then evacuated tubes have almost disappeared, I don't even know if I could price them any more. The market is telling you something. Thermal solar suffered from a one big problem: there's a reason it's hot in the summer and cold in the winter. The reason is we receive a lot less energy from the sun in the winter than in the summer. So when we need that energy the most we're getting the least of it. If you size your solar collector to be sufficient on the coldest days of winter it's going to be way oversized in the spring, fall and summer. With a thermal collector basically all you can do is vent that surplus heat. Even a large storage tank isn't enough to move meaningful amounts of heat from season to season. With PV the electricity that is produced is more versatile. If you have net metering you can just let your utility bank it for you.

    Also, I'd point out that concrete is just about the least green building material, its production creates enormous quantities of greenhouse gases. Having a large and unnecessary concrete structure in your house is not green.

  3. Jon_R | | #3

    For a watertight concrete tank, I'd look at hydrophilic waterstop in the joints. And enough joints that cracks don't occur.

    Possibly you could line the concrete tank with DIY fiberglass. Or other roofing materials (like TPO).

    Off grid without fossil fuels is a challenge. Let us know if it works.

  4. paul_wiedefeld | | #4

    Definitely a challenge on your hands: have you considered ice storage, like a Calmac tank? You could use flat solar collectors paired with a water-to-water heat pump. Instead of messing around with high water temperatures, you would use the phase change between ice and water to store more energy in less space. It would also limit heat loss to the ground since the tank would be closer to ground temperature and flat plate collectors become more efficient as water temperatures decrease. A water to water heat pump will perform well if it only has to reach 80-90 degrees for the floor.

    The most cost effective would be seem to be air-to-air and use propane/wood if needed.

  5. PAUL KUENN | | #5

    It would be nice to have a name to give you, eh? I install a lot of solar thermal but mostly for organic farms that want to heat the high tunnels. A friend in AK (Thorsten Culp - search GBA article about a year ago) has done lots of experimentation with large tank storage and it just didn't quite work because of the zone 7 cold. It does much better in zone 4-5. Our house is heated 80% with storage but for those endless cloudy days we use the Thermatech mini boiler for on demand when the storage tank water is below 110F. With a super insulated and air tight home you don't need more temp (110F) than that to stay warm. I use standard flat plate collectors that you can always find on Craig's list for a few hundred $s. Save thousands! I keep 4 on hand for all the calls I get. You can also read my GBA story about my house set up.
    Here's a new one:

  6. jj1 | | #6

    Hi User-74: Gary Reysa of has built a similar system to heat his house in a colder climate, in Bozeman MT. His DIY build was low cost and used a 500 gallon heat storage tank. See
    His complementary separate solar space and water heating system using a 165 gallon DIY tank, with an estimated budget of $2000 appears at:
    The dilemma re an off grid installation in Colorado mountain country (I own a grid connected house there) might be: adding enough capacity and storage to meet BTUs required during the longest expected time period of cold cloudy low/no sun consecutive winter days will result in a very large expensive system which is significantly oversized relative to the heating needs of "most" winter days.
    Perhaps a supplementary backup system utilizing wood or a propane heater which does not use electricity (such as the one Martin uses), could be more cost effective for those occasional extreme "long and dark and cold" time periods? (Of course, we all agree that propane is a fossil fuel.)

  7. Expert Member
    BILL WICHERS | | #7

    Have any of you that have setup systems like this used old propane tanks for hot water storage? Old propane tanks that fail their pressure testing can often be bought very cheaply, since they can no longer be used for propane, but with very low pressure heating systems they work fine. I know old propane tanks like this are popular with people who heat their homes with outdoor wood boilers. The usual thing to do is to set the tanks up vertically and insulate them with spray foam. There are no issues with this type of tank cracking.


  8. Expert Member
    NICK KEENAN | | #8

    Since you're saying the water tank is 5-10K gallons -- which is a significant range -- I assume you haven't done any engineering. Please do some engineering before you build anything. We can even help you here. The reason I want you to is that the landscape is littered with failed solar projects that could have been avoided with a little analysis beforehand.

    First step is just a Manual J so you know the basic performance of the building. Then you need to model what the tank does. I don't know of any stock methodology for doing that, but what I would do is get daily weather records for a winter that set some records for coldness. Daily solar numbers would be good as well, although you could probably get away with using historical averages.

    Set up a spreadsheet where you go through every day of the winter. For each day, take the average outside temperature and calculate the heating load. Then take the daily solar and the size and efficiency of your array and calculate solar contribution. The difference between those two either goes into the tank or comes out of it, subject to the limit that the tank can't get too hot or too cool.

    I suspect what you'll find if you do this exercise is that in order to keep from running out in mid-winter your solar array and your tank both need to be impractically large. But don't believe me, run the numbers.

  9. Robert Opaluch | | #9

    You stated "the house will not be passive designed." Did you mean that its not up to passivehaus/PHIUS levels of insulation etc. or did you mean it will not be passive solar?

    It might be a lot cheaper to build a passive solar home for wintertime heating vs. heating water and keeping it in a tank underground. Or a solar tempered (daytime) design to keep the heating load lower, so a smaller collector and tank is required. As already mentioned, active solar systems fell out of favor. Both cost and reliability/maintenance issues are problems with active solar heating systems.

    Passive solar would be cheaper than collectors & a hot water storage tank, since you can use the concrete as both thermal mass (to store solar heat gains and radiate interior heat overnight and on the rare overcast periods in CO) and as your finish floor (polished or tiled). You need to have windows anyway, and putting most on the south side of your home will give you more winter daylighting as well as providing solar heat gain. See this article for a passive solar house in Boulder, CO built 40 years ago, in a similar but colder Climate Zone 5 (-15F occasionally), vs. your 4 location:
    No heating needed on the main floor, no basement, some heat required in upstairs bedrooms on coldest nights or after overcast winter days. Cost to build is about the same as a typical house since you delete the space heating system. Also used two passive solar hot water "pre-heat" batch water tanks plumbed in series, which provided hot water, even during the winter (but only lukewarm first thing in the AM after cold nights).

    Colorado is an ideal environment for passive solar, with cold dry winters and abundant sunshine.

    Engineering a well-functioning passive solar home requires an airtight, super insulated shell (to keep winter heating needs low), and matching the area of south-facing glazing to the BTUs needed for January temps. See this article for more details:

    What are you planning for windows and window coverings? Another substantial contributor to heat loss.

  10. mhenson | | #10

    Wow, thank you for all the replies. First by the "house is not passive" is specifically solar. After spending more hours than I can count reading articles and posts on Green Building Advisor the house will be following the best green practices I can build into it. Well sealed, heat exchanged ventilation, etc. The only reason it is off-grid is because it is on top of a mountain at 8700ft and the nearest grid power is over 4 miles away. Unfortunately the house design is one my wife has her heart set on, I have tried several ideas on her to make it more passive solar to no avail. Since it is going to be double stud walls with cellulose fill it wouldn't be very expensive to make the walls and roof with heavier insulation.
    The real problem (for me) is that it would be so great to avoid putting a couple 1,000 gallon propane tanks in when there has to be a way to heat the place on-site and green. It would not offend me to put a dedicated electrical solar array up that is tilted for winter heating, and drives a heat pump or mini-splits. The problem is always night time. Heat storage for heating overnight has to be either batteries or thermal, thermal seems to be quite a bit cheaper and more green than batteries. The reason for the 5-10,000 gallon tank is because the math says 5000 will give me enough storage for two sunless days and 10K gallons is enough for four. We rarely get more than two over cast days here. I have a battery bank with a usable capacity of 23KWh battery storage that was built from used LTO batteries I bought and individually matched and tested myself. Any cell that wasn't up to spec went to the recycler. It is plenty for all household uses with several days backup, but definitely won't support a heat pump for two cold overcast days/nights. Falling back on the backup generator isn't green either. So if we accept that passive solar design isn't an option (for us). And we probably accept that a solar array dedicated to heating is better than burning propane for 20 years. What would you do to keep the house warm at night? My research on High Temp RTV silicone is that it will stand up to the heat cycling and time. Very few other materials will, almost no polymer based membranes or liquid applied polymers. If you ask why 180 degreetemps, why not lower the heat, well then the thermal storage tank would have to be way bigger, which would become too expensive as well. Maybe I am asking too much of the current technology level, but some of you guys are full blown geniuses so if you know a better way to to heat this house with my personal limitations please let me know what you would do. I am not fixated on a thermal tank so any ideas are good ideas until proven otherwise, Used propane tanks may be a viable option, once buried would they have to have some sort of sacrificial cathode to keep them from corroding or would that be unnecessary since they would be fully insulated on the outside?

  11. Expert Member
    MALCOLM TAYLOR | | #11


    A couple of decades ago I helped build a 20 ft x 20 ft concrete cistern for water storage. It leaked at the joint between the slab and walls, and also at some of the concrete ties. I spent a morning painting on a slurry of Xypex which completely solved the problem.

  12. mhenson | | #12

    My name is Matt by the way. I looked at the solar shed link provided by Jan Juran and it is obviously a proven solution. I am baffled by how he is able to do it with a 500 gallon tank. FYI this house is a little over 5K sq ft., single story. I won't go into the details of why is has to be so big but they are practical reasons. if we follow passivehaus/PHIUS insulation standards to the letter would it be possible to heat during the day and drift through the night? Then just use a high efficiency propane boiler a couple times a year?

    1. Robert Opaluch | | #13


      You probably did a lot of this already, but in case not, here's some other ideas you might find worthwhile:

      I did various sets of calculations when engineering my house:
      1. Heat loss per day on the average January day
      2. Heat loss on the coldest "design" day
      3. Solar heat gain on the January average day, fully sunny day, and overcast day.
      4. Hourly solar heat gain and heat loss on an average January day (so you see any overheating in the afternoons, and how cold the interior gets before dawn, and therefore how much supplementary heat you have to be able to deliver in the early AM especially).
      5. Heat loss and gains per room or space, not just the whole house, to balance gains and losses for each room/space.

      See this great website to calculate the solar heat gains from your windows, for your location and window orientations:
      You can find average heat generated per appliance on the web, or calculate with the appliances you are installing. Off grid this might be a small number but its likely not trivial. Average adults will give off about 350-700 BTUs/hr, depending upon activity level. Exercising heavily more like 1,000 BTUs. These all add up, especially in a very well insulated, airtight house.

      If you use a spreadsheet on your computer, you will find this to be a bit tedious at first, but a great tool for doing your design work and estimates. There are various online tools to do estimates on a whole house basis but they are not as precise nor will you get as good a feel for the dynamics and costs of making various changes to your design.

      If you get the results of those analyses (especially #1 and #2), you can determine how many BTUs you will need for the wintertime supplementary heating. Some will be provided by solar gains through windows, some will be provided by electrical equipment inside the building envelope, and a little by the occupants themselves. You can't supply all of your needs without putting most of your glazing on the south side and adding thermal mass (storage), but you can get that number to be very low if you go Passivhaus/PHIUS insulation and air-sealing levels. Or, just add more insulation and do better air-sealing to be more affordable without official PH/PHIUS certification.

      Adding more insulation in your attic would help (R-60 is good, but you can calculate the cost of adding more vs. the effect on heat loss. Once you are already up in the attic installing insulation, adding an additional amount isn't that much more material and labor cost to get it up to R-75 or so). Same for double walls, with greater spaces between those walls to get more insulation and less thermal bridging. Get more efficient windows (lower U-factor on the north especially, higher SHGC on the south especially). Think about using high-R cellular shades, window quilts, insulated shutters, or other means to reduce window heat loss affordably on winter nights. (Remember: Nighttime is two-thirds of the 24 hour days in January). You can cut your window heat loss in half most likely, without costly window upgrades beyond U=0.2/R-5. The lower you can drive your heat load, the less money you spend on those collectors and tank. And the better you survive if your system fails during winter.

      You should be calculating heat losses and gains per room or per space, not just the whole house. This is what HVAC companies are supposed to do, but don't bother, they just oversize everything. In an airtight, heavily insulated building, temps will vary less room to room. But kitchens will run warmer with all those appliances, and north-facing rooms and rooms with more windows likely cooler.

  13. PAUL KUENN | | #14

    Matt, thanks for trying all this! As for cement tanks, I get them at less than half the cost with free delivery (yes, a crane flat bed for free) from the manufacturer of septic tanks. They end up with seconds that they have to get rid of. Of course it's only free delivery up to 60 miles so nothing going from here to CO. As for size, keep in mind the first tank to heat is much smaller for the cloudy days. From there you can pump the liquid to the larger tank. A huge tank cooling down is very hard to heat up fast. At least two heat exchangers, controllers and thermistors are needed to read temps at different locations. It all does well when done right.

  14. Jon_R | | #15

    5000 gallons at 180F will deliver about 90,000 btu/hr for 48 hours to a radiant floor (a very low temp radiator in a highly insulated house). Do a Manual J, but I expect your load to be much less than this. Even less if you are willing to run the generator for a few hours here and there. With 10,000 gallons, you can use much lower tank temperatures that probably make a heat pump and plastic tanks viable.

    1. Expert Member
      NICK KEENAN | | #18

      You're assuming you can get usable heat all the way down to room temperature. One of the ways that thermodynamics works against you for heat storage is that as your temperature drops it becomes very hard to get heat out at a usable rate.

      Let's say you have a system of radiators that can distribute 90K BTU/hr at a water temp of 180F and a room temperature of 72F. When the water temperature drops to 160F the output is 73K. At 140F it's 56K. At 100F it's 23K, and at 75F it's 2.5K. Alternately, if your radiators are right-sized with 180F water, at 160F they have to be 23% bigger, at 100F they have to be 285% bigger at at 75F they have to be 36 times bigger.

      And of course, when are you going to need the maximum capacity? In the depths of winter when it's really cold, and you also need your full heating capacity.

      So you face a design challenge: do you make your radiators bigger so that you can get more heat out of the tank with cooler water, or do you make the tank bigger so you can store more hot water?

      I'm just guessing, but my gut is that the most cost-effective way is to design for a tank swing of about 30F and design your radiators so that the house can be heated with water around 110F. Size the tank so you can top it up to 140F on the coldest days.

      1. Expert Member
        NICK KEENAN | | #20

        I'll add that the same dynamic works with heat collection, the hotter your tank is the harder it is to add heat to it.

      2. Jon_R | | #29

        > You're assuming you can get usable heat all the way down to room temperature.

        If I had done that, it would be 97K BTU/hr. 94K if one uses 72F instead of ASHRAE's 68F.

        There is some question of exactly how much delta-T is needed for this radiant floor. Not much.

        1. mhenson | | #54

          A couple things that help, any time our household air temperature gets above 69F degrees I get complaints about it being too hot. If we only use the heat pump up to about 120F degrees and then use the electric heating elements won't the temperature rise according to the BTU's put into it by the elements? Also we have to de-rate heat pumps at this altitude by just shy of 27% because of the thin air. The heating elements shouldn't suffer from that or may even benefit due to less atmospheric interference. The heat pump will still be more efficient at the lower temps.
          A question for both of you, with in-slab zoned heating and sending heat to the places that need it would that be better for the very cold nights?

  15. walta100 | | #16

    Have you seen this PVC tank line rated for 200°plus?

    I looks like solar storage tank are there thing.


  16. Expert Member
    BILL WICHERS | | #17

    If you go with old propane tanks, you wouldn't bury them -- you'd put them indoors somewhere. The usual places are a basement or a seperate building built as a heat plant (which isn't usually all that big of a building). You then have to insulate the tanks, which is best done with a heavy coat of spray foam, but some people have just wrapped the tanks in batts.

    I think your best option is to use a backup generator system. This is the usual way to backup an off-grid solar system. The reason for this is that you are right about the greeness of batteries, and batteries are both a big cost and an ongoing maintenance item -- they have a finite life. I would put in a propane tank, a small propane generator (Kohler makes good units for this purpose), and either run heat pumps or keep a backup propane furnace. A system like this lets you size your battery plant for a shorter period of cloudy days without compromising your home's functioning. The tradeoff in terms of greeness is some propane use on the rare occassions that your solar system is underperforming. Note that you can actually calculate how "rare" of an event this is too. Propane has the advantage of lasting indefinetely in storage, so if you go years between tank fills you don't have to worry about the fuel going bad. You are also prepared for any unusual weather events, like a 2 week cloudy period or something that would be very expensive to build for with a solar system. I think this is a safer option for you with very little downside.

    You could potentially use a wood boiler as supplemental heat for your primary solar heat source, but you still have the issue of running circulation pumps during extended cloudy periods, and that means you need electricity. A generator solves this issue. I don't really see a better option for this if you want to go for 4, 5+ days of cloudy tolerance in your system. Remember too that if you get a period of severe weather, you could potentially exhaust your solar capacity to make power, and now you're stuck with no heat and no power while you're potentially stranded waiting for a major storm to pass and rods to get cleared. A propane backup system offers you safety and insurance against unsual events like this, and that propane system just sits there costing nothing and burning nothing unless it's absolutely needed.

    BTW, with an off-grid system, you can set things so that the generator runs at a near optimum efficiency point (often around 80% of capacity), charges the batteries, then shuts off until the batteries drain down to some threshold that you set that begins the next charging cycle. This means the generator doesn't have to run continously, and ensures that when it is running, you're using that propane as efficiently as possible which means you are getting the most useful energy out per unit fuel in as possible -- the greenest way to run a system. I helped someone in Northern California with a system like this a few years ago, and that system was built entirely with scavenged parts (I sent leftover wire from one of my projects to help the guy out).

    Anyway, I would recommend beefing up your insulation as much as reasonably possible, sizing your solar system for a reasonable amount of cloudy days (maybe 2 days?), then using a backup generator for any extended periods of solar underproduction. I think this is your most cost effective option, very green since you're doing most of the "work" with your solar system, and the safest in terms of being able to cope with unusual weather events in the future.


    1. mhenson | | #19

      I really do appreciate the concern, we lived off-grid for about 3 years in the early 2000's. The in place electric system is 16KW, 8kw inverter, 12KW propane generator, 23KWh battery, full 500 gallon propane tank. We will be adding a second 8KW inverter once the house is built. My main goal here's to try and build it as close to the ideal as is possible.
      So if we go with it may be impossible to get 365 days a year off solar. The heat will only be needed on the stormiest days/nights of the year, which will probably be very cold it seems a back up HE propane boiler would beat a generator running a heat pump in emergency mode. I am going to follow Jon's advice and get a manual J value. If a smaller tank will work for thermal storage I won't mind above ground in a small building. I will also check out the link for that liner.

      1. Jon_R | | #30

        > as close to the ideal as is possible
        > a back up HE propane boiler would beat a generator

        If you recover the heat from the generator, it will provide electricity and heat. On the other hand, generators have high purchase and maintenance costs. And the hope is that running it is so infrequent that it makes little difference.

        1. mhenson | | #55

          Exactly, while I may not make it. The goal is for the generator to only be on for its weekly exercise and the propane boiler to never fire-up.

      2. Expert Member
        BILL WICHERS | | #33

        Remember that the boiler will need electricity to run circulation pumps, and possibly an inducer blower. That's the upside with a small generator -- you get electricity whenever you need it, regardless of the weather. I agree a propane fired boiler is likely to get you more heat value out of a unit of propane, but the downside is it doesn't get you electricity to run anything else, and probably requires electricity to run itself.

        Jon mentions heat recovery from the generator. I've looked into that before, and it's expensive to build. Chances are it will be too expensive to justify considering that it probably won't get used very often. Generator maintenance costs aren't very high. Typical maintenance is an oil change every year or 100 hours of operation, whichever comes first. An oil change is probably aroung $30-40 for oil and a filter.

        There is a lot of information about thermal storage in water tanks on, with the guys that use wood boilers to heat their homes. I recommend you read through some of the forums over there to get some more info about this. Typical tanks are 500-2,000 gallons or so, and the goal is usually to store enough heat that you only have to fire your boiler once per day, with the storage taking care of the heating needs for the rest of the day. Your application is similar, but possible longer duration.

        BTW, earlier you were interested in ice. Any time there is a phase change (solid to liquid, or liquid to gas, typically), you have a big step in the amount of energy involved. This is why steam is more efficient than hot water for heating, and why you can store more energy "in reverse" with ice than with cold water. There are some phase change thermal materials for passive homes, although I'm not aware of any being in common use.


  17. Expert Member
    NICK KEENAN | | #21

    The greenest source of heat is probably wood. A lot of people run wood-fired boilers with tanks like this, the problem with wood burning is it's hard to modulate the output, having a big tank to soak up some of the excess makes it work a lot better. I could see sharing the same tank between solar and a wood boiler. That way you don't have to size the solar for the worst possible case, instead you can size it for whatever you consider a reasonable portion of the time and plan on burning wood the rest of the time. Most people size the tank for a wood system to basically hold one day's worth of heat, that way you only have to stoke the stove once a day and can do things like leaving the house or sleeping without the house getting too cold. I'd size a solar tank similarly.

    If wood isn't for you (it isn't for everyone) then propane is probably next best.

    1. mhenson | | #57

      I will put chimney flu in the mechanical room, since I am making it larger i will create an area to install an appropriately sized wood burning boiler. If we don't reach the goal of "generator only exercising and the propane boiler never firing", then I will install a wood boiler and use that when we are home. We certainly have plenty of firewood here.

  18. PAUL KUENN | | #22

    Matt, other choices:
    I believe these folks sell the hot water liners as well for very large tanks.
    As for controllers for pumps:

  19. Expert Member
    NICK KEENAN | | #23

    Here's a question I have: if your goal is to create hot water from solar panels, which gives the most hot water at the lowest cost:
    * Evacuated tube solar collectors
    * PV through an inverter powering an air-to-water heat pump
    * PV DC out powering resistance heat

    Each has advantages and disadvantages. DC out is the only one where efficiency doesn't drop as the water temperature increases. It's also the simplest. Heat pump will get the most heat from a given area of panels. The PV panels produce electricity which is useful for all sorts of things other than heating water. Evacuated tubes can produce hotter water than a heat pump.

  20. mhenson | | #24

    Great info so far, my original plan was to put the evacuated tube panels in a loop filled with industrial heat transfer fluid. It is human safe, rated from -57 to +700 degrees and with a properly sized pressure tank to keep it in the system. The fluid is very expensive, but it would only take about 8 gallons to fill it. However, with a larger solar system the electricity has a lot more uses and a lower price tag. So I am benefiting from smarter people intend to buy another solar array. Unfortunately, the hottest output from the Arctic heat pump is 125F (The manufacturer told me 135F degrees first then later said 125F). I will probably get one anyway to use for cooling in summer and maybe DHW in the summer. I don't know if you can set it up to pull the heat our of the house/slab and put it in the DHW tank, but it sounds elegant. A few more valves sounds easy until you think about balancing it. Researching the exploitation of the phase change heat difference on ice. Never heard of it, but can't wait to learn more. The list of new ideas and hardware/technology options is growing and it is great. As to the three ways to heat, Inverters, heat pumps, et. are expensive to replace and or maintain, so for this project PV DC resistance heating of the water makes the most sense to me (so far), also diverting DC from the charge controllers isn't any more difficult than programing the heat pump to turn on. I will probably turn off the thermal storage system in the summer and use all that power for a Tesla or something.

    1. Robert Opaluch | | #26

      Once there is a charging station reasonably close to your (unspecified) location, you might use your electric vehicle as a power supply limited battery backup. The upcoming Ford F-150 electric was designed partially for this purpose.

      In your sunny location in CO, when the temps are very cold, the skies are clear. That's how it gets so cold, the warmth of the earth is radiated out to space overnight. So it matters how you design your house, the cardinal direction of your windows, window glazing area, and window SHGC of south-facing windows especially. Your solar heat gain mid-winter in Colorado is substantial and would be a partial backup heating system even if your home is not designed to be passive solar to store those gains. To some extent, even if you have an equal amount of glazing facing all directions. During midday and afternoons, you should not need supplementary heating if skies are mostly clear, and you have a clear view to the south especially. You might want to look at solar insolation charts and cloudiness charts at to get a better idea of the wintertime solar gain potential of your glazing, unless you live in a forested area, north facing slope, or are shaded by adjacent buildings to the south.

      At least do approximate calculations of your solar heat gain from windows. If you lived in a more northerly location or in an area of overcast skies in winter, this would not be very relevant. But in your geography, it is an important factor to consider. For you, this solar heat gain mid-winter (especially in the coldest periods) can be significant. And its free heating if you exploit it.

      The cost of going passive house/PHIUS is substantial, but in your off-grid location, it might be worth it. It is likely to be lower cost than the system you are proposing. As others have already noted, active solar systems have not worked out well due to maintenance issues. And in your case, it appears you already face design challenges and cost issues.

      Multiple heating plans might work out when combined.
      1. Increased insulation and air sealing not quite to passivhaus/PHIUS levels (get your heat load down as low as affordably possible);
      2. Solar heat gain from your south-facing windows (but not to the level of passive solar);
      3. An affordable electric vehicle or smaller backup battery system;
      4. a small propane backup heater; and
      5. a more affordable tank with solar collectors for you to conduct your heat storage experiment.

      If you have not calculated your heat loss and solar gain numbers, you could post your details here and maybe some of us can do some rough estimates or guide you to online resources to help get that done. If you have run the numbers, please post them here to give us a better idea of the scale of your space heating shortfall.

      Note that in that passive solar house I built, the coldest period dropped to -15F overnight, and never got to zero F for days. Clear skies overnight and cold air from Canada made that happen. The interior went up to the low 70's by afternoon, and dropped to 65 by dawn, without using any supplementary heating system, and with insulation levels a bit lower than yours, and before the days of all these new air-sealing products. The worst period was a three day storm with overcast skies all day (once in five years). Interior temps slowly went down to 59F without using any backup heating, but that was primarily due to the thermal mass of the 4" concrete slab/tiled floor and significant slab edge insulation. The cost of this system was net zero, since it eliminated the central heating system and used that money for increased insulation and air-sealing labor. Mid-winter electric space heating cost for the upstairs bedrooms was $35/mo in today's dollars and main living area downstairs $0. Your climate appears to be warmer but similar, but we don't know your location and site characteristics.

  21. GBA Editor
    Martin Holladay | | #25

    1. The value of the thermal heat that you collect will be too small to justify the high cost of the hardware you need to buy to collect that heat.

    2. Maintenance problems will consume far more of your time and money than you now think.

    For decades, people have been trying the scheme you propose and failing. For a detailed story about one such failure, read this article: "Revisiting the Sunrise House in Fairbanks."

    1. Robert Opaluch | | #27


      Its true that in general, ACTIVE solar heating systems have not worked out well compared to alternatives, due to costs and maintenance headaches. And passive solar CAN fail to perform well if not designed well. Some people don't bother calculating heat gains and losses, and built houses based on guesswork, that overheat, get too cold due to insufficient insulation and air-sealing, or experience glare from interior contrast colors, etc. You like to cite examples of those failures.

      Oil and gas heating systems can kill people from backdrafts, fires or even explosions. Not to mention contributing to killing the planet for everyone else.

      Fairbanks is the most severe winter climate I can imagine for a city. IMHO Chlupp deserves kudos for his experiment, which didn't completely meet others expectations and was not cost-effective. I'd call it a daring experiment to push the limits of design in an extreme climate. Colorado's winter climate is a piece of cake compared to Alaska. Or for most Colorado locations, benign compared to your own northern Vermont with often overcast winter skies. Matt's experiment might work out, but I wouldn't try it myself, for the reasons you cite. Since he appears to be committed to trying this, I hope he shares his results here, showing us what worked well and what didn't.

    2. Expert Member
      NICK KEENAN | | #31

      Martin --

      I agree with you. I would go on to say that most of these attempts were never properly engineered, if someone had run the numbers to begin with they would have seen they don't pencil out. As I said earlier, there's a reason it's cold in the winter and hot in the summer. To collect, store and distribute enough heat in the coldest part of the winter requires an enormous capacity that is largely unused in the rest of the year.

      I'll add that passive schemes suffer from similar fundamental flaws. The problem is that sunlight is at its lowest in December and peaks in June, while temperature lags by about six weeks. So March and September have the same amount of sunlight yet September is typically about 20 degrees warmer. Where I live September is cooling season and March is heating season.

      It's easy to fool yourself. Today is a late fall day, it's clear as a bell and should get up around 50. Stand by a window with the afternoon sun pouring in and it's easy to think, "I don't need heat." But it's a different story at midnight in January.

    3. mhenson | | #48

      Hi Martin,
      I have read a lot of your posts and articles and respect your opinions very much. Assuming you have my limitations on not being able to build a purely passive house and that I want to make the house as green/efficient as possible. Also, it is very remote, bringing in large amounts of propane will be expensive and can only be done during warm months (the propane truck can't negotiate the road in winter). What would you do? I don't mind spending a bit of money on experimentation. Maybe it would be smarter to size the tank well for the manual J and then put in a small dedicated array and/or a small evacuated tube array and then use propane to make up the difference and then up size what works best? I got fixated on the evacuated tubes because my mother-in-law has a couple that were on the house she bought three years ago. They only do DHW and love them, almost zero maintenance over the last three years. I thought using an industrial heat transfer fluid and tubes rated to -59F and plus 700F then covering all but a couple panels in the summer would make it fairly maintenance free. But you know your stuff better far better than I do. So I am very interested to know how you would approach this project.

  22. wastl | | #28

    Move into a hotel nearby for a few days if the weather does not line up (once in a while)??

    Edit: the one above is called "out-sourcing" and not that unusual today..

    If you want to pursue a active solar home then
    - this is achievable in your cz!
    - start with a superinsulated (passive) base design, heating load at design point in the range 10 to (max) 15k Btu/h
    - add a proper solar storage tank inside of the house - forget something outside of the thermal boundary - too much heat loss
    - 5000 gallons - maybe more - that depends on your climate - how long without sun etc.

    have a look at - that is one of the european solar pioneers with a proven success record. A multifamily house (8 apartments) with 100% solar heating + hot water supply - it is doable!
    It depends whether this is worth for you - if you will live the next 30 years in that house then why not..

  23. mhenson | | #32

    I lived in Soldotna AK for a couple years as a teenager, that climate isn't even close to CO. It is way colder, if it isn't raining it is snowing, and I can remember never seeing the sun in winter because we were inside the school before it came up and it was down before school let out. The closest town to our location is Canon City, CO but we are NW of there and a lot higher. So Buena Vista has about the same climate, but we don't have mountains blocking the sun to the west. I don't want to give the exact address for privacy reasons but we are up Hwy 9 then off county rd. 20 in Fremont County CO.
    I have insolation data that the evacuated tube manufacturer provided. However it is based on Canon City at 8000 feet which is 700ft too low (attached). We are in a meadow right on top of the ridge so will get early sun and keep it until late in the day. I have gone back and forth on putting the tank inside and am now leaning hard toward one 500-800 gallon DIY interior tank with an already planned DHW storage tank probably in the 120 gallon range. I already sent a quote request to Witt Liners. I recognize that active solar has more cost and maintenance, but really believe it can be done in this climate. I am not so naive as to ignore the math or to not have backups. Frankly it doesn't get much more simple than a dedicated solar array tilted for optimum winter sun going to a heating element inside the tank. In this configuration a Victron Energy 450v/200 amp charge controller is the only real hardware between the array and the heating element. Yes, there will be a water pump or pumps, but they will be battery and generator backed and easily replacable. The tank being inside the building may be much more practical, though I might end up having to change the design for a larger mechanical room. I now have a lot of engineering homework to do.
    One other thing that just hit me.
    Does anyone have numbers on just the expense the glazing, sealing , and heat loss through that glazing, installation costs of heat storage mass etc. of a passive house It would be very cool to compare to my actual costs of a solar array, charge controller copper, pumps and storage tank.

    1. Robert Opaluch | | #34


      I'm not sure what you are asking when you say "passive house." That can mean Passivhaus/PHIUS which is a very strict low-energy usage building, requiring very high insulation levels, high end windows, and HRV/ERV, resulting in a very low heat load. The target is to reduce heating load by 90%. It can have a heating element in the fresh air delivery system and otherwise no other space heating is needed (except some solar gain, appliance "waste" heat and heat from occupants). Certification requires a substantial insulation budget and buying their approved highly energy efficient window options, as well as hiring PH experts to help design and evaluate your home.

      Or you could mean Passive Solar, which requires a highly insulated and airtight envelope to keep the heating load low, but not to PH/PHIUS levels (not as expensive). Passive solar uses solar heat gain as the main source of space heating, and also requires substantial thermal mass (typically a 4" slab floor, polished or tiled typically). Thermal mass stabilizes interior temps and stores heat passively to keep temps reasonable into the next day or through cloudy days. Interior temps would vary daily from 68F to 78F if well engineered. The building must have a clear view of the southern sky so sunlight is not blocked from 9 or 10AM to 2 or 3PM during Dec-Jan, when the sun is low on the southern horizon. Typically that means any buildings, stands of trees, etc on the south side of the building are twice as far from the house as their height. Typically that mean the majority of the glazing area faces south, with a window wall or large windows on a building that is longer on the south side (say 48' wide) than the east/west sides (say 24' wide). The solar energy is "free heat" but its not often that you can find a site and climate that works well for passive solar. And you need to calculate solar gains and heat losses to make them balance out mid-winter. Thermal mass will keep interior temps stable in summer too, but you must minimize or shade west and east-facing windows to avoid solar gain midsummer. The house tends to run warmer in the fall, and cooler in the spring, due to seasonal temperature lag behind the solar calendar. Nothing stopping you from using some backup heating, but it likely will be minimal during successive overcast days or in the early AM.

      Both of these eliminate a central heating system to save costs that are put into higher insulation levels. Passive solar can't work in cloudy winter climates or far north climates. See tables in this article for more:

      Colorado tends to be ideal for passive solar. Or anywhere with cold, dry/sunny winters that are not extremely cold.

      Passivhaus was designed for Northern European climates and this standard is now required for new construction in Belgium and parts of Germany. The US version is PHIUS, basically the same standard but takes into account the regional construction costs and climate to adjust design goals.

      You do not have to adhere to these approaches fully to get the benefit of their approaches. "Pretty Good House" is a loose term that refers to higher than code requirements for infiltration/air-sealing, insulation levels, and perhaps solar orientation when available, and tends to avoid any fossil-fuel heating systems, yet keeps the budget for these items lower with no real constraints or standards. Usually that means a minisplit for heating/AC per floor. Its sensible construction given what we now know about the importance of air-sealing, higher insulation levels and indoor air quality.

      A solar-tempered house tries to capitalize on wintertime solar heat gains, but lacks the thermal mass (e.g., slab floor) to store heat for overnight or overcast days. Typically that means no winter space heating mid-day into the evening, but not through the night into morning, or only working best on mostly sunny days. Some backup heating is essential, probably mini split (or in your off-grid case, propane).

      You might consider these approaches and still try your solar heating/tank experiment. After all, a mini split would be require more power than you have off-grid.

    2. Expert Member
      NICK KEENAN | | #35

      The spreadsheet helps.

      Some calculations you can do:

      You had mentioned that you were considering 17 arrays of tubes. In January, each array produces an average of 49.5kBTU/day with reflectors, which gives 843kBTU per day or 35.1kBTU/hr in a 24-hour day. Your January average high is 48.5F, low is 19.7F which gives an average of 34.1F. You really need to get a Manual J done, which will tell you what your 99th percentile temperature is, but for the sake of illustration I'm going to guess. Around here the 99th percentile is around 5 degrees below the January average low, so I'm going to use 15F. To maintain an indoor temperature of 72F, a house uses 1.5 times as much energy at 15F as it does at 34F. If your house has enough solar capacity on average to get through an average January day -- 35.1 kBTU/hr at 34F -- at 15F the load can't be more than 52.8kBTU/hr. So that's the first thing to look at in your Manual J, if the design load is over 52.8K then it's game over, you don't have enough solar to keep warm in the winter.

      Now that solar doesn't land continuously during the day. It's probably a safe assumption to say it all comes in an 8-hour window. So during those eight hours you have to bank enough to warm the house for the next 16 hours -- at 35.1kBTU/hr that's 562kBTU. Let's say your tank has a usable swing of 30F. To bank that much heat you'd need 18,700 pounds of water, or 2257 gallons.

      That gets you through an average day. What about peak days? This all depends upon your assumptions. What's your worst case? It's customary to use 99th percentile temperatures in the Manual J, but 1% of a year is 87 hours. Around here when we get a cold snap we get days where the average for the day is around the 99th percentile temperature. But your weather could be worse. What if you get a 3-day blizzard that completely covers the solar collectors, and on the evening of the third day a cold front moves in and it drops down to -5F? Should the tank be designed to hold a day's worth of heat? Three days? A week? I don't know, but that's something you have to figure out.

      1. Robert Opaluch | | #43

        DC, Matt,

        Typically for passive solar you design for the average January (coldest month) temps and average wintertime solar gains. On the coldest "design" days, the shortfall either lowers the interior temp, or you use backup electric heat to stay more comfortable (but of course not much heat is needed, just the difference between current day and average Jan conditions).

        Passive solar has thermal mass to reduce this drop in interior temps by "borrowing" from the thermal mass, then replenishing on days afterward. Solar-tempered homes don't have the thermal mass and use "backup" heating system energy most winter overnights. During partly or fully sunny midday into evenings, solar gain does the job in solar-tempered homes.

        For Matt's off-grid situation, rather than take the costlier alternative of designing for the coldest "design" day, it still makes economic sense to design for the average January day. But that means some type of auxiliary heating system is needed for the most severe winter weather (e.g., as suggested already, propane backup heating, days in a hotel, battery power, electric vehicle backup power, etc).

        My experience is the most severe winter weather for solar homes is extended overcast periods, rather than the design temp (which is reached with clear skies typically). That's why passive solar doesn't work in cold climates with mostly overcast days (e.g., Pacific Northwest, Buffalo NY). Calculating the heat losses and solar gains would show the results for your location. I've calculated gains expected on sunny, average sun, and overcast January days, and compared to heat losses on design, average and warmest January temps. Can give you a good idea of the challenges you face and the backup heating you need for various conditions. Even then, I wouldn't design for the coldest temps AND overcast conditions because they never or very rarely happen. And your home doesn't go to outdoor temps for days if left with no heating at all. Many days for super insulated, airtight homes.

  24. mhenson | | #36

    I don't know if it is ok to ask, but can someone PM me to recommend someone I can pay to help me with the manual J. I would like to compare the added expense of the glazing, window sealing, 4ft of concrete etc. to what I am going to attempt.

    I am not trying to do a passively heated home, if you read above this will be a "pretty good home" I am probably going to up the insulation in the roof to R80 and the walls to R50 or 60 depending on the cost/benefit of the heating system.
    I did some basic calculations on a small electric solar array below that will be dedicated to space heating and tilted for maximum benefit at winter sun angle.

    8Kw solar array minus line loss to charge controller and heating element, loss to charge controller and loss from controller to heating element and efficiency of the heating element itself. I calculate to be about 7% in my system. Using 8 percent just because my calculations aren’t always perfect.
    If I put in an 16Kw array there should be 14.6kW per hour or 43.8kWH per day. Then my primary array should have about 12kW per day of surplus, so 55.8kW per day directly to the water. If only three hours of full sun at the dead of winter (and there will average more than that here). This would supply 55.8kW of heat to the tank per day. That is 190,397,523.6 BTU’s of heat to the tank or 7,933,230.15 per hour for 24 hours (yes, ludicrous). So really I need the manual J in order to size the array and the tank. However, my estimate on the cost of this array and interior tank with wiring and hardware is below $25K.
    I hope this doesn't upset anyone, but unless someone shows a flaw, it appears I have a "pretty good" solution that will work. With good insulation "pretty good" normal size windows and a hydronic 4 inch slab it may be possible to get to the 97th+ percentile on pretty low carbon and solar panels. I honestly don't mind being totally wrong about any of this. So please shoot holes in it.
    So far you guys have convinced me to:
    Drop the evacuated tube array and replace it with electric solar panels (array size will depend on manual J).
    Drop the exterior heat storage tank and use a smaller tank inside the building envelope.
    Use a tank membrane that I didn't know existed
    Limit the tank to no more than 1000 gallons (or two touching 500 gallon at a maximum of 180F degrees (tank size will depend on manual J). Also the slab will likely need to be thicker under the water tank(s) to bear the weight.
    Use an on-demand HE propane boiler as a backup (wood would be the preferred, but it needs to automatically kick in if we are gone for an extended time).
    Maybe put in a heat pump or some mini-splits for summer cooling (if we even need cooling).
    Up the amount of insulation in roof and walls depending on how it affects the manual J
    Lowered the overall cost of my project by an estimated $40K.
    Maybe up the insulation under the slab to a full 6 inches of rigid foam (depending on Manual J).
    Locate the insulated thermal storage tank(s) in the north side/end of the house.

    Solar panels do use more resources to make than passive glazing, and there is a short run of 10ga wire from the panels to the controller 1.02% voltage drop, the charge controller will be pretty efficient at just dropping the 425+DC voltage from the array to 48v DC for the heating element. Pex in the slab and a few other things. Overall not very carbon heavy... I didn't realize Passive Haus had 4ft thick slabs of concrete. Everything else in this house would have been installed anyway. Yes it is an actively heated home, but I am now very excited to build it.
    I will respond to further comments, but probably won't bother you guys until I get the manual J completed, which will probably be in January.

    1. Expert Member
      Akos | | #37

      I have a cottage with an a 10kW array in snow country (Zone 6). It is not uncommon to go through a week where the array produces 250W peak when a snow storm moves in. Relying on sun to heat in snow country is problematic at best, you are lucky if it produces enough power to run your house loads.

      There is a big difference between statistical minimum insolation day and reality.

      1. mhenson | | #51

        Hi Akos,

        I had a house just done the road from this one with a 9kW array and only a 8kWH battery bank. We had plenty of power, but the battery was always charged well before noon and we would run out of battery on near the end of overcast days the genny would kick on. With 16kW of array and 23kWH of usable storage and with better southern exposure we should be close to my estimates. The saying here is "if you don't like the weather wait 10 minutes". Our storms tend to blow in drop a lot of snow and then it is clear and cold the next day, same with rain, we do get a lot of flash floods and hail. The array is also ground mounted as we have over 50 acres and it is a lot easier to clean them.

    2. Expert Member
      NICK KEENAN | | #38

      1 kWhr= 3412 BTU. So 55kWhr= 187,000 BTU or 7,819 BTU/hr over 24 hours.

      1. mhenson | | #40

        Big oops, I accidentally multiplied by watts not kW. However, that may still work fine. I can change the array size. Also it may have happened, but I can't remember a time in the last decade when we had more than three overcast days in a row.

  25. badmechanic | | #39

    Have you considered a thin stainless steel or aluminum liner?

  26. mhenson | | #41

    Since the tank will be in the house in the end it will just look like a wood box. However, I was planning to buy something aluminum or stainless if the DIY liner doesn't work or is more expensive. The link is an example of one 600 gallon tank, they have many sizes.

    1. wastl | | #42

      Nice tank. Did you see that one:
      vertical (more efficient stratification and use of floor space) and easier to insulate (also a tad bigger),
      and more affordable.
      If that one is too high, rather build a pit in the floor to accomodate the height.

      Ref. your electric system: if the power control etc. is in the house then the voltage drop and other losses are already heating up the house. You might want to look at theses losses not that negative, but yes - for the storage they are lost.
      If you are limited in collector area, then a vaccum tube or flat collector will still provide more bang, although the PV is more versatile.
      Did you ever check to use a heat pump for the hot water (if you go PV)? Will the collectors be able to provide the hot water as a primary system and propane as backup?

  27. Robert Opaluch | | #44

    Note that if you move the water tank indoors, your home moves closer to being passive solar. You don't need to pump as much water to distribute heat in your home. Heat will migrate from your tank to the surrounding interior area. And you could locate south-facing windows/glazing or skylights for direct solar gains to heat the water tank with no pipes or pump (passive gains).

    Passive solar requires substantial thermal mass, which can be from a 4" concrete slab floor, or stone, brick, concrete walls that are insulated from the outdoor temps. Clear tubes of water have been used as well, but they aren't attractive or conventional looking, so were a niche "hippie" passive solar home product. But water has more thermal mass per weight than the other alternatives, so its a good thermal mass material, except its liquid, so can't function as a building component (floor, wall).

    1. Expert Member
      NICK KEENAN | | #46

      Longtime readers of this site know that I don't like it when people use the term "thermal mass," it's not a term that is used in science or engineering. What we're really talking about is heat capacity. Generally when people talk about "thermal mass" they're under a couple of misconceptions: first, that houses are somehow lacking in heat capacity, and second, that adding concrete is a good way of adding heat capacity.

      A typical four-bedroom house weighs on the order of 100,000 pounds. And it's built of materials that have a higher specific heat than concrete -- drywall, tile and even wood all have higher specific heats. Most houses already have plenty of heat capacity. So why doesn't that capacity buffer swings in temperature? Why does your house get hot on a sunny day and chilly on a cold night? It's not because of a lack of heat capacity. It has to do with heat transfer rates -- heat can't flow in and out of the solid parts of a house fast enough to be an effective buffer. Modeling those transfers is complicated stuff, but I use a simple formula that I learned from John Seigenthaler, which says that the heat flow from a surface into a room is given by:
      BTU/hr/sf=0.71*(temperature difference in Fahrenheit)

      So if you have a surface that's five degrees from ambient, the flow is about 3.5 BTU/hr/sf. If you're trying to achieve heat flows of tens of thousands of BTU/hr -- if you look at my post #35 above I model that we need to collect 70K BTU/hr during sunny periods -- you need tens of thousands of square feet of surfaces. Alternately, you need much larger temperature swings. But large swings create problems of their own, both with occupant comfort -- and the whole point of heating is comfort -- but also with efficiency, as the house gets hotter it loses more and more heat to the environment. So the problem with houses isn't that they don't have enough heat capacity, but they don't have a way of getting heat in and out. So Matt is actually on the right path with trying to store heat in hot water and then redistribute the heat using radiators. (I don't think it's going to work as proposed, but for other reasons.)

      Notwithstanding the fact that most houses have no need for additional heat capacity, I particularly want to warn people off from using concrete in an attempt to increase their heat capacity. First, it has just about the lowest specific heat of any material commonly used in construction. It seems to be a common misconception that density is a good predictor of heat capacity, and yes concrete is dense. But that's meaningless. If you really needed to increase the heat capacity of your home you'd be much better off doing a double layer of drywall, you'd get more heat capacity and the sound attenuation benefit of thicker drywall. Concrete is also a lousy surface for a residence, my feet, knees and back ache if I spend too long standing on a concrete floor, it's unforgiving if you fall on it or drop something breakable on it. If it gets damaged there's no real way to repair it. And it's ugly. But most important, it's environmentally irresponsible to use concrete where it isn't necessary. The production of concrete is a leading source of greenhouse gases, we should be cutting it out wherever we can.

      1. Jon_R | | #49

        +1 on this. You won't get much heat into passive thermal storage without overheating the space. So I'd design the system to use heat from the water tank every night.

      2. mhenson | | #50

        Excellent information thank you. During the cold season my plan is to use the tank to distribute heat every night. If for some reason there isn't enough heat in the tank the propane boiler will automatically make up the difference directly to the floor. I could just install an HE propane boiler, a couple thousand gallon propane tanks and be done with it. However, that takes all the fun and challenge out of trying a greener solution. For practical purposes is might make the most sense to use a heat pump to get the temp up to 120-125F degrees and then switch to the heating element to push it the rest of the way up. The previous two houses I had, had radiant slabs and we really liked them...we did break several things. We did put some really nice spring-loaded rubber mats in front of the sink and my workbench. I will investigate the double drywall, to reduce the size of the tank and/or increase the thermal capacity of the house. Please tell me the other reasons you don't think it will work. I realize that no one will ever use this as a model for a tract home. But this is the last home we will ever live in so the payback period isn't as important to me as it would be for a developer. As as side note, my wife was in a car accident (that wasn't her fault) last year and is now in a very heavy power wheelchair, concrete has a benefit for us in that regard, cleanup, wear-and-tear, etc. we can't have carpet, we could get away with high wear layer LVP, but it would still have to be replaced sooner than the average. I am still a few months away from breaking ground and most everything is still fluid, so I will take any ideas you share seriously.

        1. Expert Member
          NICK KEENAN | | #59

          " I will investigate the double drywall, to reduce the size of the tank and/or increase the thermal capacity of the house. "

          The point of my long rant is you don't need to increase the heat capacity of the house. You're better off putting resources into your tank.

          1. mhenson | | #60

            Got it, just used your calculations on how fast heat moves out of the drywall.

      3. Robert Opaluch | | #52

        DC Contrarian,

        Okay instead of calling the 4" slab "thermal mass", you could call it "thermal storage", or what would you prefer or consider a more accurate portrayal? Its an integral part of a system to capture energy from the sun, and store heat to radiate back into the interior when the sun is not shining and providing interior winter heating energy.

        Explain to me why the downstairs area with a tiled 4" slab has typical daily temp variations +/- 5 degrees F, vs. a bedroom with about the same area of drywall as the downstairs slab (about 25% double 1/2" drywall, 75% single 1/2" drywall) has temp variations about 50% larger, despite having about only about 25% of the glazing area of the downstairs tiled slab. Homes framed with wood with interior drywall finishes do not work to store enough thermal energy and radiate it back effectively enough. What's the terminology you want to use here?

        Or are you saying that if you put 8 layers of drywall you would see the same thermal energy storage, radiation and effective interior temperature damping? If so, you are still using materials whose only function is energy management. A floor slab is your floor system as well (serves double duty), stone or brick wall functions as a wall, as well as working as a part of an interior winter heating "system" that is fueled by energy from sunlight. Sunlight is the most sustainable an energy source as you can get, and its free. And your windows are part of the same integral space heating system, and are required in a functional home as well. Nice in several ways.

        Another advantage is that its passive. No pumps, no electricity usage, no maintenance, a bit more sustainable. Not affected by electrical outages.

        The system works with a 4" slab, polished or tiled. Or with comparable area of brick or stone. Note that most homes have a basement slab of the same area and volume, plus thicker 8' high concrete walls. That's much more material usage than just a first floor 4" concrete slab (raft slab, thickened edge slab). And basements do not work as wintertime home heating system, and are uglier than the tiled or polished slab you diss on aesthetics.

        The biggest drawback is that passive solar or solar tempered homes don't work in all climates (e.g., overcast winters, extremely cold far north winters, or benign climates like Key West, San Diego or Honolulu). Or on building sites that don't provide a view to the south for wintertime solar heating.

        Solar tempered homes (warmed by the sun late AM through early evening) do not use or need that thermal storage component. However you get the free sustainable heating and daylighting for only about a third of the day, but it saves a significant amount of energy. You do have to have a compatible building site, orient the building to capture sunlight through windows facing south/southeast/southwest, and have a backup heating system for overnight and overcast wintery days.

        Nor is solar tempered or passive solar buildings the only way to build a sustainable, energy efficient building. Just one choice for certain climates and sites.

        Free sustainable heating and daylighting with some design work and constraints. You'd think people would jump at the chance when they have the opportunity. But natural gas is a more popular option.

        1. Expert Member
          NICK KEENAN | | #56

          "Explain to me why the downstairs area with a tiled 4" slab has typical daily temp variations +/- 5 degrees F, vs. a bedroom with about the same area of drywall as the downstairs slab (about 25% double 1/2" drywall, 75% single 1/2" drywall) has temp variations about 50% larger, despite having about only about 25% of the glazing area of the downstairs tiled slab."

          I've never been in your house so I can't explain what's going on, there's lots of reasons why houses heat unevenly, most do to some extent. The only way to know for sure that the concrete slab is the source of the behavior would be to rebuild the house without it and see if the behavior changes. But I have no reason to believe that the concrete slab is behaving the way you seem to think it is, because there's no science to support it.

          "Homes framed with wood with interior drywall finishes do not work to store enough thermal energy and radiate it back effectively enough."

          Let me ask you this: what physical property does concrete have that drywall lacks that makes the statement above true?

          1. Robert Opaluch | | #74

            Concrete has lower R-value to move heat more easily. Drywall has modest R-value, as does wood. Adjacent air or sunlight striking the surface will move heat into concrete, stone, brick and water more easily. Heat will move more readily from concrete, stone, brick and water into the interior air than drywall or wood.

          2. Robert Opaluch | | #80

            The main difference between the upstairs rooms and the downstairs open floor plan is the floor. Upstairs, carpet on 3/4" ply subfloor. Downstairs, ceramic tile on 4" concrete slab. Also there are 3 bedrooms up, mostly one big open area down. So more drywall and wood framing upstairs, less down. Ratio of glazing area to floor area is comparable.

        2. Expert Member
          NICK KEENAN | | #58

          In terms of nomenclature --
          The physical property of matter that we're talking about here is "heat capacity." (see ). The quantity that is moving around is "heat."

          I don't like "thermal mass" because it's pseudo-science. In physics and engineering "mass" has a very specific meaning, the resistance to acceleration when a force is applied. Newton defined mass as force divided by acceleration, F/a. Now the word "mass" is used in architecture differntly, to refer to the size of something. When an architect refers to a building as "low-mass" he's not saying he put it on a scale and it doesn't weigh very much, he's saying it doesn't have much of a visible physical presence. I'm pretty sure the term "thermal mass" was coined by somebody with an architecture background.

          "Thermal" is also problematic. When you learn physics one of the things that is drummed into you is the difference between heat and temperature. While colloquially "thermal" is used for both (and the terms themselves can be used somewhat interchangeably) in science thermal refers to temperature. When an engineer talks about a "thermal gradient" in a structure, he means one side is hotter than the other; when a piece of metal is subject to "thermal stress" it's because it's not all at the same temperature. But we're not talking about storing temperature -- if that were even possible* -- we're talking about storing heat. So the structure has heat capacity, heat storage is what happens and heat is the quantity that flows.

          Using scientific-sounding words -- and using them improperly -- is a hallmark of pseudo-science.

          *(It would not surprise me at all if the person who coined the phrase "thermal mass" believed that storing temperature was a thing.)

          1. wastl | | #66

            I trust the difference is not the specific heat capacity.
            The slab will typically "see" the sun, it has the (absolute) capacity and a good conductivity that the solar energy is passed into the whole mass so that the temperature rise is rather minimal.
            A drywall is rather thin (even double wall) so that that same energy will lead to a higher surfave temperature and thus overheating.
            A concrete wall/floor above 3-4" thickness does not help you because the temperature gain is low and the wall will not have that overtemperature you need to drive the energy back into the room over night.

          2. Robert Opaluch | | #75

            "Using scientific-sounding words--and using them improperly--is a hallmark of pseudo-science."
            1. If you don’t like physics terminology being used in this context, please suggest a better term or terms for the use of concrete, rock, brick or water in passive solar buildings to store heat energy and release that heat later, and dampen interior heat fluctuations. Heat sink? Heat storage? Solar heat gain capacitor? Heat absorption and re-radiation component? Seriously, I’d like to find a better term, despite its common usage in passive solar design. Especially the term “mass.” We don’t care about the weight or size, we want the “energy absorption, storage, and release” concept and “temperature dampening” concept in one or few clearly understood words.
            2. There are many words which have different meanings in different contexts in the English language. That causes confusion, I agree. Language can be confusing, especially the English language with its adoption of words from many sources.
            3. Not liking the terminology doesn't imply the thing doesn't work, or its scientifically disproven a theory, or it cannot be engineered well, or should be ignored. I assume this was not your intent, but others reading your comment might believe so.

      4. Expert Member
        BILL WICHERS | | #53

        Yeah, we actually do use the term "thermal mass" in engineering, but generally only when referring to a solid thing, an in my wold (electrical engineering), that usually means a heatsink. A simple example would be a heatsink being able to dissipate x watts in continous operation, but being able to "absorb" short bursts of much more based on the mass of the heatsink, which is calculated using the specific heat of whatever material the heatsink is made of (usually aluminum). You can work out how many joules of energy it takes to heat the aluminum itself up some number of degrees, and knowing the safe max temperature of your device, you can work out the mass of heaatsink needed to support some short duration heat "dissipation" (not really dissipation in this case). The heat sink's ability to dissipate that heat then sets the "cool down" time, which limits your cycle time for your gizmo (the cycle time is how long you have to wait before you can run another operating cycle for whatever part is making heat).

        I did all this some years back to build a circuit breaker tester that needed to dissipate very large (10s of kw) amounts of energy for a period of only a few seconds. The device needed to be portable, and didn't have to cycle frequently, so the heatsink was only a few pounds or so. You'd plug it in, trip a breaker, get a readout of the time to trip, then the device would sit -- and cool off -- while you documented things before running the next test.

        I have myself used the termal "thermal mass" to mean the volume of water in a buffer tank, and everyone knows what I mean when I say that. It's probably a bit of terminology appropriation, but the term has essentially been accepted by most these days even if not entirely correct.


        1. Expert Member
          NICK KEENAN | | #64

          OK. What are the units of thermal mass?

          1. Robert Opaluch | | #76

            A combination of R-value and heat storage capacity. Glass has low R-value but can't store much heat. High R-value or even moderate R-value materials like wood and drywall don't work well as a thermal storage and release material for storing passive solar direct gain energy and releasing it back into the surrounding interior air.

      5. johngfc | | #63

        Very nice explanation!

  28. Jon_R | | #45

    Here is a web site where you can do your own Manual J load calc:

    You really need at least a rough value to determine your tank size. I suspect that hundreds of gallons is too small.

    I'm interested in what is less expensive - more PV with resistance heat or less PV with to-water heat pumps (you probably need several to move enough heat during the short sunny periods). The former can use a smaller, hotter tank - but a bigger tank probably isn't that much more expensive.

  29. mhenson | | #47

    Great website thank you. I can model different insulation values and see where the sweet spot will be with regard to the cost of the solar array and tank size. For the tank itself I will probably go with a box/rectangular shaped stainless steel, put a layer of heat resistant insulation and then poly iso sheets, and yes as vertical as possible. So far the ceiling height is intended to be 10ft.

    1. Expert Member
      NICK KEENAN | | #61

      The design of a system like this is going to be highly iterative. First find out how much heating you're going to need, from the Manual J. Then decide out what kind of radiation you're going to have, whether it's radiant floors, radiators or convectors, you can mix and match. Then figure out how much of each type. Then figure out what water temperature you need to get the heat output you need with the radiation you have. Then figure out how you're going to heat the water. Then figure out how big a solar array you need, and how much water storage. And so on.

      At any point if the number you're getting seems impractical, go back to an earlier step and revise it. For example, you could get to sizing the solar array and realize it's bigger than you want, and at that point go back to the Manual J and add more insulation to lower the heating load. Keep doing this until you have the whole system figured out.

  30. Expert Member
    NICK KEENAN | | #62

    A couple of thoughts:
    Bill had the suggestion in post #33 to go to and look at the heat storage systems people use for woodburning. That is an excellent idea. As your system evolves it seems to be converging on something similar to what those folks do, basically trying to store one day's worth of heat.

    Don't feel you have to do the whole house in radiant flooring, one of the nice things about hydronics is you can mix and match radiant floor, traditional radiators and convectors.

    My gut is that the most practical configuration is a PV array with inverters powering an air-to-water heat pump. That means water temperatures probably 120F and below, whatever the sweet spot for the heat pump you choose is. It also means a lot more radiators than hotter water.

    I know the Chiltrix heat pumps will automatically switch to resistance heating when the COP drops below 1.0.

    If you have the choice between a small amount of warmer water or a larger amount of cooler water, the warmer water is better. What some of the woodburners do is have two tanks, one with cold water and one with hot water. They start out with the hot tank empty, and they pump cold water out of the cold tank, heat it, and send it to the hot tank. If they need heat they take water out of the hot tank, use it and return it to the cold tank. That way they don't have to wait for a full tank to get hot water.

    1. mhenson | | #65

      I very much agree on the iterative nature of this design process. I waited over a year to make my first post because I wanted to be close enough to the actual ground breaking to not be wasting other people's time. Here is my iterative struggle with heat pumps:
      They have to run through the entire electrical system and before you get any benefit you lose 20%+ or so to the charge controllers, inverters etc.
      Then because of our altitude and the lower density air you have to de-rate by another 27%
      (based on the Fujitsu table in this link ). Some may be better than others, but air density is air density.
      They are only as good as the COP they are able to produce, with a COP of 2 after the solar system and the altitude there isn't a vast improvement over emergency mode (COP 1).
      Running the heat pump at COP 1 doesn't make sense (to me so far) since you still lose so much to the inverters etc.
      So running a heat pump when it is at or above COP 2 would be efficient and below that probably not (for my location and setup). Running a 48v diversion load directly from the charge controller to a 48v heating element or elements in the actual water tank does not lose anything through the inverters has no de-rated loss due to altitude and doesn't loose efficiency as the temperature in the tank goes up. So above 120F inside the tank it would be close to a true COP 1 all the way up to 180F degrees.
      As to solar panels, it will mostly be iteratively modeling the cost of panels vs. insulation vs. how much heat can be stored in the size tank we put in the house.
      I built a detached garage (30"x50" with pre-engineered storage trusses so lots of storage upstairs) at my current home based on what I learned from this site. It has 4 inches of rigid foam on the outside, a vented roof, insulated slab, is very well sealed, and has no windows. I heat it with a little portable electric element heater I stole from our bathroom. It has not once dropped to freezing inside and usually I don't even have to put on a sweater when working out there on normal winter days. The construction was agonizing and I won't do it again. Double stud walls and cellulose from now on, or until someone comes up with something better.

  31. wastl | | #67

    I apologize that my inner engineer is sticking its head out, no lecture is intended!
    Assuming that you will build a larger one level house the passive solar option has to deal with the fact that the sun will not reach to the northern part of the house.

    Considering off-grid and that 100% solar will be over the top in terms of expense maybe a compromise?
    - backup propane or wood stove + propane generator
    - the south side leaning towards passive solar and
    - radiant floor north side and MAYBE south side, design temp as low as practical (a "cold" system temp. will increase the stored energy in a buffer tank) Also you could run the south part through the day to transfer heat from the sun into the other rooms (a "cozy" floor temp system will not do that)
    - a buffer tank for the hydronic rated for 2-3 days of "no sun" - not necessarily coldest day (because of propane) to keep the size down
    - buffer tank - now the fun part :
    - heated by PV + heat pump or
    - solar collectors for the best efficiency / sf area (we talk about cold sunny days here - not shoulder season - PV+HP would win) - but would be big enough also for the shoulder season. I guess that we talk about >200 sf area here to make that system work in the normal winter times.
    - propane boiler as the backup (or a wood stove)
    -if the buffer tank is larger and slim enough (good stratification) you could use the hot water on top to run a heat exchanger for hot water on demand. Expect to need maybe 5-8°F buffer tank water temp. above the required hot water temp. Obviously you have to make sure how to load and how to use the buffer tank water- if you mix the water volume up all that energy is wasted
    A wood stove would be able to dump one run into that buffer with no sweat, use return flow boost valves and load to the top to avoid creosote buildup and have the hot water topped up.

    Design wise - do the math before you commit yourself. (it was stated here already enough) The balance point between "how much solar" and "backup", either you define a buffer size/collector size and that will give the time the house could run on stored energy (for a given ambient air temp/sun profile) or you define the climate you want and sizes are the result. Obviously all that is a combination of all factors - including the house/thermal envelope/heat loss etc.. So - better do the math first..

    Yes, this a nerd´s system and any installer will run crying away if you ask them to service this. YOU will be the senior engineer.. If you break a leg then pray that the system will work. That is what would let me hesitate to do it. If you are fit and will get the return out of that for the next 30 years then? If properly designed and following KISS then also chances of failure are reduced - but - it is something to consider.
    Otoh, there is nothing like rocket science involved.

    regards and all the best

  32. mhenson | | #68

    Thank you Bas, I am prepared to fail on some level, if you read above there will be a propane boiler. In fact if I was really after a cheap non-green solution propane tanks and a boiler would do it.
    You can also read the challenges of heat pumps run by inverters and at our altitude. I know this can work. May not reach the goal of "generator only running weekly exercise and propane boiler never firing up", but I think I can get very close. Where I am right now is a heat pump, dedicated solar array, heat storage inside the building, and lots of insulation, with a very well sealed building envelope. I am working on a manual J by iteratively changing the model between all of those things to find the sweet spot for size of array, size of storage tank, and amount of insulation. I think the most important thing for me is realizing the heat pump won't do very much for me, and direct heating elements in the storage tank will. With over 20% loss to inverters etc. before the power even reaches the heat pump and then losing 27% of the heat pump efficiency to altitude, then since the heat pump can only produce 125F degree water and it loses efficiency as the temp goes up in the tank it doesn't offer much. BUT, once the tank is at 120-125F degrees sending all the solar at 48v DC from the charge controller and bypassing the inverters and other losses, the heating elements will put more total BTU's into the tank, also since the efficiency of a heating element doesn't doesn't decline as the temperature rises hitting 180F should be systemically simple and at the same time effective. I really don't mind building and supporting it myself, but here in Colorado there are a LOT of professionals with enthusiasm for this sort of project. Also most of it will be basic electrical and plumbing:

    Pumps can all be mounted on the same wall and changed out with the flip of two valves and an adjustable wrench.
    The solar arrays and system will be completely standard including the 2-wire starting generator and the oversized battery banks. Well except for the fact that the battery is made up of large Lithium Titanium Oxide cells, that I assembled myself.
    Heat pumps are very standardized now, though this one is less commonly used since it is air to water.
    The solar to heat storage tank will be KISS, the charge controller will use the diversion 48v output to run to the heating elements in the tank no different than a domestic hot water heater just a much much larger tank.
    The propane boiler will be an on-demand HE boiler which is very standard.
    The plumbing wall that directs all of it will be a bit complex, but any skilled plumber should be able to read all the labels on all the pipes, mixing valves, etc. and make sense of it.
    With all the help I have received from all the people in this thread, to my mind at this point, the only hard hurdle left is balancing the sizes of the array, vs the size of the heat storage tank or tank system, vs the output from the heat pump, vs the amount of insulation. I can't succeed or fail at that until I know much more about the heat loss of the building which is what I am working on now.
    Currently I am cautiously optimistic as I have an idea of how many other projects like this have failed, but very excited to work the numbers. This may all look very different in a month, but one way or the other it looks doable. Just don't know if I can reach the penultimate goal of not using any fossil or wood fuel (except exercising the generator for 15 minutes a week).

    1. Expert Member
      NICK KEENAN | | #69

      I'm trying to find a definitive answer on this, but it seems like heat pump capacity is de-rated at high altitude, but not efficiency. So you need a bigger unit than would at sea level but the COP is the same.

      1. wastl | | #72

        High altitude - reduced density - less capacity/efficiency of the condensor for a given deltaT? The capacity is reduced and the lift is a bit higher. So - efficiency is a tad lower but not that drastically. A bigger condensor / fan would have to be mated to the otherwise same sized HP. Due to that it is a air-water system you pay for the altitude only on one side.

    2. wastl | | #73

      Ok - good answer. Actually the inverter (if within the house) would not "waste" the energy... What you lose is the chance to use that lost energy to power the HP with the better COP and just heat the house...
      Still, some vacuum collectors would also give you that high temperature capabilty, just saying...

      Also, I do not know why you need 180°F. Hot Water - 150°F is enough, same with space heating. Fpor daily swings especially with a HP, better have a slightly larger tank at a reduced temp.. (my opinion) 150°F with an R32 HP is just doable..

  33. Expert Member
    NICK KEENAN | | #70

    This page has a COP chart for the Chiltrix air-to-water heat pump (just for the sake of example):

    If the earlier spreadsheet you posted is correct that your average January high is 48 and average of 34 then most of the time it will be running in the 34-48 range (it won't be running at night). In that temperature range the COP is over 3.0. So you would have to have over 67% losses for resistance heating to make more sense.

    They don't say what output water temperature they're assuming.

  34. Expert Member
    NICK KEENAN | | #71

    OK, here's my attempt at a comparison of PV+inverters+heat pump vs. PV+resistance heat vs. evactuated tubes:

    PV+inverters+heat pump:
    I'm going to use Chiltrix because they have pricing info on their website. Their heat pump is rated 2.7 tons heating, COP of 3, costs $4,689. But based on the charts on their website I think it's good for about 2 tons heating at the temperatures we're talking about here. So let's say 24K BTU/hr. With a COP of 3 that's 8K BTU/hr of input energy. With 20% inverter losses that's 10K BTU/hr of solar production, or 2.9kW. Now you have to oversize the solar collectors to get capacity with less than 100% sunshine, I'm going to guess by twice (that may be low). * So we're looking at a 6 kW solar system. Last time I priced a solar array it was right around $3 an installed Watt, so $18K, total of $22.7K. I figure you'll get four usable hours a day so around 100K BTU/day.
    *(Note that with grid-tied solar and net metering you wouldn't have to do this, you could utilize 100% of the solar production. The same 100K BTU/day would take 4kW of installed capacity, costing $12k)

    Resistance is 100% efficient (COP of 1) so for the same 100K BTU/day you need 100K BTU of electricity or 29.3kWhr per day. Using the figures from post #36 you get 2.7Whr per installed Watt. So you need just over 10kW installed. This comes to $30K, less probably $3K for the inverters, or $27K. You could use a cheap electric water heater which shouldn't be more than $500. So let's say $27.5K total.

    Evacuated tube:
    From post #32, a 30-tube array with reflectors provides a claimed 49,596 BTU/day in January at this location.
    For the same 100k BTU/day, you'd need two 30-tube arrays. Looking at they are about $1500 each, so $3K.

    I really need someone to check my numbers, because you could knock me over with a feather, these results are the exact opposite of what I would have expected.

    In the original posting Matt had said 17 evacuated tube collectors were necessary. So installed cost looks to be:
    PV+inverters+heat pump: 9 units @$22.7, or $204K
    PV+resistance heat: 8.5 units @27.5, or $233K
    Evacuated tube: 17 units @ $1500 or $25.5K

    1. Expert Member
      NICK KEENAN | | #83

      Just for completeness, I want to compare with what you'd get with a grid-attached system.

      First, you wouldn't have to store up heat during the day. So instead of having to produce 24 hours worth of heat in the eight hours when the sun is shining, you could run your heat pumps 24 hours a day. So you'd need 1/3 as many heat pumps, 3 units instead of 9.

      Second, instead of having to produce enough solar each day to provide heating for that day, you'd only have to produce enough electricity in a year for that year. From the insolation data in post #32 January provides 6% of the solar for the year. Googling degree-day information I get that in Denver January accounts for 18% of the degree-days of the year. So by averaging you only need 1/3 the size of the solar array.

      Putting them together you end up with a 18 kW array and three $4.7K heat pumps, total is $68K at $3/Watt installed, $41K at $1.50/Watt installed.

      But if you're not storing water there's no need to use a air-to-water heat pump, using air-to-air would probably be half as much, which knocks it down to $34K at $1.50/Watt.

    2. Expert Member
      NICK KEENAN | | #84

      One more comparison:
      Using batteries to store energy instead of water.

      Amazon sells a 6KW solar system with 19.2 kWh of battery storage for $12,600:

      You'd need 9 of them plus 3 2-ton heatpumps. About $130K if you stayed with air-to-water, about $122K if you went with air-to-air.

      1. Expert Member
        NICK KEENAN | | #86

        Based on this I now think it's cheaper to store your energy in batteries than in hot water. Certainly electricity has a lot more potential uses than hot water does.

  35. mhenson | | #77

    Wow, I figured all the calculations that lead me to evacuated solar panels had somehow been based on a false premise or I had made a mistake on my numbers. The Duda panels were the ones I was planning to use and the SRCC certification ratings are very good. They also come with the ground mount so besides anchoring them it is pretty straight forward. I still plan to put the tank inside the house. In either case I will super insulate the pipes from the panels to the tank. Also Duda has stainless steel flex pipe that is very affordable to go from the panels directly into the tank. They also have a controller that allows you to set the temperature of the heat transfer fluid and manages flow. I will probably use this fluid from the panels to the coil exchange in the tank.
    It won't boil if the temp of the fluid goes to the boiling point of water for a short period.
    It doesn't breakdown and turn acid over time like glycol based fluids.
    It will only take about 8 gallons so it is very affordable for this project.
    Sitting in the pipes over the summer with the panels covered won't have any known deleterious effects.
    It only goes up to 425F degrees, but since the panels will be covered all summer there is little danger of stagnation in the winter. The other possible option would be to pressurize the heat storage tank at a low pressure and put in redundant LARGE relief valves that have a very large vent through the roof, in the exceptionally rare case (in fall or spring) when the panels might over-heat, the water would just boil at 195F degrees and actually cool the panel headers preventing overheating. My estimates for the Duda panels (with help from their sales person) installed with controller, pump, insulated lines to the tank, transfer fluid, pressure tank, misc hardware and a 10% surplus for miscalculations came out to $34,442. Once I get my tax credit they are very affordable (yes they qualify). The only electricity it will use is the controller (with sensors) and the circulation pump and since they are cheap I would put in redundant circulation pumps, negligible cost really.
    One other thing to like about the evacuated tubes is because they are tubes they don't have to be anchored to the ground with lots of steel, concrete, or both because the wind blows around the tubes. There is little to no uplift like there is with electric panels.
    Please shoot holes in these ideas. I hope Martin will read these last two posts and find the flaw or flaws. I need someone who doesn't like these types of active systems to really try to break it.

    1. Expert Member
      NICK KEENAN | | #78

      So one apples-to-oranges comparison is that for the PV I'm using installed cost and for the evacuated tubes just the cost of the components. Component cost for the PV is probably closer to $1.50 per watt with inverters and $1.20 without.

      Also, with the evacuated tubes when you produce surplus it's a problem that you have to deal with. With PV it's usable electricity. Some value should be assigned to that.

      I'm also a little dubious of the claimed production of the evacuated tubes. A Canadian Solar 400W panel is 2m x 1m, ysing the figure of 2.7 Whr per installed watt gives 540 Whr per square meter per day. It's rated at 19.9% efficiency which implies insolation of 2700 Whr per square meter per day.

      The Duda panel is 2mx2.25m or 4.5 square meters. They claim 49,596 BTU per day, or 14,500 Whr per day, or 3229 Whr per square meter per day. That implies 119% efficiency! I'm using their number with the reflector but even without the reflector you get 104% efficiency.

      1. mhenson | | #79

        Hi DC,
        There are several reasons evacuated tubes are much more efficient. One is they are always "tilted" at the exact angle to the sun because the heat pipes in the tubes are round, they also pick up sun/light from all directions, reflection off the tube next door, reflection off the ground etc. also evacuated tubes lose almost nothing to convection, conduction, etc. because they are in "vacuum tubes" Their ratings are independently SRCC rated, but if I had to put an 18th panel out there...ok. Also remember electric solar panels require a photon to displace an electron in a piece of silicon, so the most advanced electric solar panels are about 23% efficient, while a black tube will absorb almost every single photon that hits it. They really are apples and oranges as you stated. Please read my latest post and shoot holes in it.

      2. Expert Member
        NICK KEENAN | | #82

        The last time I did a thorough comparison of evacuated tubes and PV was in 2013. I just went back and re-read my notes and I also used Duda tubes then, and I assumed 27% efficiency at 47F. I had linked to my sources but they are dead now.

        Looking at the Duda website, they don't talk much about efficiency. The only discussion I could find is here:

        They provide the formula:
        η = 0.420 - 0.6544(P/G) - 0.00310(P2/G)

        where η is the overall efficiency.
        0.420 is the baseline efficiency, efficiency with no temperature delta
        P is the temperature difference between the collector and ambient (no units given)
        G is the insolation in Watts/m2.
        So we're probably talking about quadrupling the size of the collector, pushing the cost over $100k.

    2. Expert Member
      NICK KEENAN | | #81

      With the heat transfer fluid you're going to need a heat exchanger, which my calculations don't include.

      The thing you run into in this business is that every time you go to exchange heat, the smaller the temperature delta the bigger your exchanger has to be. So you're going to want to run your hydronics quite a bit cooler than the fluid coming off the roof in order to have a reasonably sized exchanger.

  36. Jon_R | | #85

    One can purchase PV panels for $.36/W. Possibly $1/W (vs $3/W) is enough to get them installed and connected to a resistance heat element? Maybe evacuated tubes should have a similar markup for installation?

  37. mhenson | | #87

    You both bring up very good points. The one that concerns me the most is the efficiency of the panels. Because they are in vacuum we know they are significantly more efficient than flat plate panels that drop of quickly in cold weather. I am not as concerned with the ET (can I abbreviate Evacuated Tube?), but do need much better numbers. If I do have to double the amount of panels it would extend the payback period by a lot. I would have to start balancing how many days a year the propane boiler would have to run vs. number of installed panels.

    With regard to the heat exchange, my plan there up to this point is to run the same inexpensive stainless steel flex pipe that Duda sells for the panels in a multi-loop inside the tank, effectively making the tank the heat exchange. Then run normal supply into and out of the tank. The exchange rate etc. has to be calculated, but it wouldn't offend me if I had to run a 100ft of 3/4" stainless flex pipe inside the tank. At 0.028 gallons per foot 100ft of hose would displace less than 3 gallons of the volume and not really lose anything to efficiency.

    As to installation of the ET panels, that cost is negligible. They will be ground mounted on the mounts included in the price of the panels. So it should be as simple as digging a shallow trench the same dimensions as a railroad tie. Set railroad ties into the trench and attach the mounts to the ties. I probably will use a greener solution than railroad ties, but it doesn't take much to hold down ET panels at at 110 mph wind load. Code in our county is 100 mph. If there was real concern, I could also stake some additional strapping. My expectation is that ground mounting will run me less than $1500 total, even if I have to rent a trencher or small hoe for the crew.

  38. Robert Opaluch | | #88

    Given the very high rough cost estimates I'm reading here for this wintertime solar space heating system...
    1. You might consider hiring a PHIUS Certified Passive House Consultant to help design a very low heating load building:
    They can help minimize air infiltration, thermal bridging, and your overall space heating energy usage, with more expertise than any of us here. Although their fees are not cheap, the costs of your heating system might be reduced by their well-engineered design suggestions. They also provide a more reliable and detailed estimate of your heat load.
    2. Start estimating your heating load with numbers like attic R-80, walls R-50, triple pane windows, $2-3k passive house entry doors. These options might reduce your heating load to cut your heating system costs more than the costs of these upgrades. And provide some "insurance" if your system needs maintenance mid-winter.
    3. Maybe think about exterior shutters that could protect your windows in those windstorms, as well as provide some insulation value when used?

    1. Expert Member
      NICK KEENAN | | #89

      You definitely want to make this house as tight and well-insulated as possible. The insulation levels in the IRC reflect rules-of-thumb about when insulation stops being cost-effective, but they're based upon typical cost of energy. In effect your energy is a lot more expensive than typical, so a lot more insulation is cost-effective.

      I'm not sure you need a PHIUS consultant, just look at how they build houses in places a couple zones colder than you.

      Note that all types of systems benefit equally from increased efficiency so the decision calculus doesn't really change.

  39. Expert Member
    Akos | | #90

    Besides the design work, a couple of pointers.

    For ground mounted panels make sure they are high enough off the ground so the bottom doesn't get covered with snow. This is not just snow fall but snow sliding off the panel, which can be a more than snow on the ground.

    Go for smaller MPPTs and wire the panels in a way that some partial shading doesn't take out the whole array. For example, the bottom row tends to be most covered with snow, so that row should be on its own MPPT.

    For your heating system, always think about how to simplify the design. Any time you add a pump, you need power to run it and controls for it. More failure points. I would read through the blog here for a good start:

    There are also other ways to simplify the system, for example go for an LP water tank with a built in heat exchanger coil (ie Bock EZ 75-76 PDVLP-C or Laars Combi). You can simply plumb the space heat and DHW output of your storage MegaTank to the inputs of the LP tank. This way if the MegaTank outputs get too cold, the LP tank will automatically kick in to provide the extra heat. This now all happens without any extra pumps or controls.

    Since the LP tank can do both DHW and space heat, you don't need a separate LB boiler for your space heat.

    Fully heated slab will not get you the warm toes feeling in a low load building, especially if you are looking for tile cover (tile is also pretty rough on the feet, I would reconsider that choice). The issue is that your space heat loads are low enough that the floor will never get warm enough to be felt, at best it feels not cold.

    What you want is to only heat part of the slab with high traffic as now these can run much warmer. Even better, skip the fully heated slab and go for some panel rads, in a well insulated house, there is practically no comfort difference.

    Inverters can fail and generators won't start. Make sure to have a plan to heat the house without power (ie through the wall propane heater). I would also keep all the plumbing and heating in a central location as it is easier to condition.

    Make sure your septic and water lines are well bellow frost lines. Running heat rope to keep pipes from freezing takes a lot of power.

  40. mhenson | | #91

    The R-value in the roof and walls will be balanced against all the heat inputs, the heat storage, and etc. We had already budgeted triple pane windows, R-80 attick and R50 walls seems like it may be over-kill for this project, but the numbers will prove it out one way or the other.
    But yes, double stud walls well sealed on the outside, modern framing techniques, fully insulated slab, footers, and stemwall. Also we will put runners on the roof and vent the entire roof from the eves to the ridge-vent.
    I very much like the idea of a propane water heater/exchanger over a boiler. Will look at it carefully when we get to that point.
    The nice thing about our solar location is that there is a sloping hill to the south of the house, all the panels can be mounted down that hill and as snow slides off it will continue to slide down the hill. So we don't have to mount the panels so high off the ground.
    As to in-slab pex or radiators, or fan blown radiators. I have lived in a heated slab home and we did like it. The concrete never got warm per se, but if you walked around barefoot it was comfortable rather than cold. My only experience with radiators was as a kid hour school was heated with big steel radiators. I remember we had to be careful because sometimes they were hot enough to burn you. I am sure modern temperature controls and design have solved that, I mostly don't understand what the difference is in terms of heating benefits of one over the other? Sorry for my ignorance but what do panel rads do that make them better, simpler, or more efficient for this situation?

    1. Expert Member
      NICK KEENAN | | #92

      Radiators are a whole lot cheaper than radiant floors for the same amount of heating. They're a lot more space efficient, which is great in rooms like bathrooms. They're also more versatile. With a radiant floor, if it turns out you goofed in your calculations and you don't have enough heat, there's only so much you can do. You can turn up the water temperature, but only so far until it gets uncomfortable. With a radiator you can turn up the water a lot hotter. You can also pretty easily replace it with a bigger radiator.

      The beauty of hydronics is you can have radiant floors, radiators and convectors all in the same system. So you might do radiant floors in the living room and master suite, radiators in the guest bedrooms and convectors in the basement.

  41. PAUL KUENN | | #93

    I think you guys are reading and thinking too much. This is real world usage in NE WI where we are below zero F on many days with weeks of clouds. With 4 flat plate (1980s) collectors, dc variable speed (Ecocirc) pump and exterior (to tank) heat exchanger, we keep 110F temps all night long in two tanks (50 gal & 70gal) for in floor heating (4 loops at 250' each; 1200sq ft. house). At 150F tank temp which we get on somewhat cloudy days (170F+ tank temp on blue sky days), a heat dump is used to warm the basement. Keep in mind we have much less sunlight than you. I know because I built an earth home above Boulder Co in the early 80s. Twice for kicks and grins, we turned off the heat completely in January during cold spells when day time highs never reached -5F below zero and the house never dropped below 58F in three days of the test. I do know I did an excellent job of super insulating the 1960 2x4 house.

    1. Expert Member
      NICK KEENAN | | #94

      I read through the four pages of the blog. The insulation job is truly impressive. But on page 4 you say you're average 21.9 kWhr/day of electricity usage. Is that net of your solar? Because that's equivalent to about 100,000 BTU/day, which for a house that well-insulated is a significant amount of heat.

  42. onslow | | #95

    Over a decade ago, before moving to the Colorado mountains myself, I also heard the siren song of extended thermal storage. Martin is correct, in his terse Yankee way, of simply advising against the idea of seasonal heat banking. Many a project has crashed on the rocks attempting this. It seems that you now are past the idea of a 10,000 gallon tank and see your goals as a diurnal heat banking effort. This is a more manageable goal for which there are many examples. The overall successes of these efforts are still variable when balancing costs and complexities against results.

    For many individuals, the most manageable way of setting aside sunshine energy for later use is growing and cutting down trees for firewood. This works pretty well when the energy demands are personal and purely local. Your concern over reducing your impact on the environment by avoiding fossil fuels is appropriate and should be everyone's universal concern. Unfortunately, our collective energy demands have been badly managed. We seem intent on using the sunshine energy set aside long ago by burning every bit of coal, oil and gas we can extract. We have already consumed several million summers of sunlight and seem determined to burn the entire planet before we wise up.

    Here on the GBA site, ideas for how to minimize impact on the environment while still providing individuals their desired comforts generates a lively breadth of opinions. You have already received a wide sample of very good advice for the question asked. However, it may be best to set aside the original question of how to best construct a thermal energy holding tank and instead reset the question.

    Which technologies and engineering can affordably be utilized to effectively (and hopefully efficiently) heat your home in your particular location. (I say heat, as your elevation of 8700' likely precludes the need for AC.)

    Pre-selecting any one method of energy management to the exclusion of others generally tends to result in non-optimal outcomes. Many of the posts back to you have already made the same point. You display an engineer's approach, so do review all your options dispassionately and with KISS principle in mind. Mo'complexity means mo'problems to paraphrase a pop culture meme.

    First off, I am not sure your basic assumption of being in CZ4 is correct. Canon City is actually at an elevation of 5331' making your site difference over 3300' not 700'. Have you lived or camped on the site? Some of your posts sound like you have, others not so much. Are your present calculations based on long term data at your build site? I ask, as it is just hard for me to imagine a January mean temperature of 35F at 8700' anywhere in Colorado, despite the large daily temperature swings we have.

    I live at 8000' due west of you in, I think, a very similar climate context. My experience suggests your actual energy needs would likely be higher than my own. When doing my own calculations I used building climate zone 6B, 7200 HDD and a design temp of -10F, which has proven to be a pretty fair match to my actual energy needs over the last 5 years.

    A quick back of the envelope estimate of your heat load at 0F for a basic box of 5000 s.f. with 2550 s.f. of 10' walls, 450 s.f. R6 windows and doors at 70F delta T plus 5000 s.f. floor slab R20 with 20F delta T totals to about 20,500BTU/hr or 492,000 BTU/day. Heat only. That is 144 kWh of energy you need to heat for ONE 24 hour zero day. PV Watts calculator projects a 16 kW array will produce an average of about 45 kWh a day in January. And you still have your non-heat daily load to add in. Even a highly efficient low temp air to air heat pump with a COP of 2+ will consume more than you produce.

    For comparison, I have cove heaters (Long story in other posts) , an 80 gallon Marathon water heater (the Pig), all electric appliances, 90% LED lighting and slightly lower insulation than you seem to have planned. My summer base average is inexplicably 25 KWh per day, my peak winter daily average is 100 KWh, so I infer that my peak per day winter heating load is 75 KWh minimum.

    The house is over 4000 s.f. on three levels with the basement shop included. My whole wall R is 36+ and whole roof R is 52+ and my air exchange is unknown, but seemingly small. My wife and I argue over the thermostat at 70 - I am the cold one.

    As I am in the middle of calculating a new build near me, I will offer some other comparables as I can. Meantime, a few things to note from the many posts. Mr. Opaluch, Mr. Kuenn and DC among others have been offering much good insight. I agree with DC that battery may still be a better path for storage. Remember a tank of hot water is just another type of battery with fairly high losses depending on insulation levels. You can at least benefit from the losses if you put the tank in your house, but still it is "leaking" heat 24/7. As DC noted your thermal value drops as you access it which changes your distribution needs and there are limits on how far down you can drain the "battery".

    Also, note that stainless steel is generally not a good conductor of heat and thus not the best choice for in tank heat exchanger. Copper is over 10 times more conductive. A tank of 1000 gallons weighs over 4 tons so plan on under slab support to prevent possible slab movement. 1000 gallons of water will cover 5000 s.f. of floor a bit over a 1/4" deep. Have you tested the soil profile for expansive clays or shales?

    Good triple pane with gas windows can range into $75+ a square foot. Per square foot, small windows cost more than big windows. Expect to pay more for operable windows. Fixed windows have better U values. You will not need as many windows for ventilation as you may think. Try to get your doors into protected alcoves or under porch roofs. Thermal shades can cause condensation problems. Solar gain is a two edge sword, consult with Mr. Opaluch for summer heat management.

  43. mhenson | | #96

    Thank you Roger,
    I will attempt to address most of your questions and look forward to your responses. Duda diesel used Canon City for the LAT/LONG and then used 8000ft as the altitude. So we are actually 700 feet above their estimate.
    I did live very close to the property just down the hill about 1/2 mile away and about 300 feet lower. I was only there for three years, but did run all our household electric from a 9kW solar array, a very small 8kWH LA battery, our heat was mostly from a wood fired boiler and an HE on demand propane water heater that fed two 120 gallon heat exchange tanks. I designed and built that system with the help of a good plumber and a local solar guy. It worked very well except the battery was very undersized ( it would usually be charged by 10 or 11am and if we had a cloudy day the generator would start up at about 3-4am). I couldn't afford more at the time. The floor was hydronic/concrete, but only insulated with 2 inches of ridged foam. I was a novice, but learned a lot from that project.
    I am not worried about our household electrical usage our array is twice as big and our battery has a USABLE storage of 23kWH.
    We had completely given up on trying to heat with electric solar panels until DC gave us some more numbers, the evacuated tube panels (ET panels or ETPs?) still look viable to some degree as well.
    I do regularly throw out ideas that are not viable, but do hold on to what I learned from the exercise. The idea I am currently playing with is:
    Putting in a backup propane heating solution no matter what, because if all else fails it will keep the house warm when we are away, so that is a sunk cost.
    I am probably going to put a large insulated heat storage tank in the house, for whatever green heating solution works to whatever extent is works even if it is a small one for a wood burner. I am going to install an 8-inch chimney into the mechanical room with space allocated underneath it to install a wood burner of some sort, but may not buy a wood boiler until it is verified to be needed.
    It may be wiser to buy about three or five ET panels and evaluate their real-world output, it is easy to add many more if they are viable, and if not those would be more than enough for year-round DHW with any excess dumping into the heat storage tank. We really liked our concrete floors in the previous house so that is probably going to stay. If I go the evaluation route with the ET panels, then I will probably install a single heat pump to evaluate that as well, but also if it won't work for space heating it will work for cooling in the summer. We usually had a few weeks in the heat of the summer when the house temp went up over 80F degrees, and it was way hotter outside so opening windows wouldn't and didn't help. That house had R20 walls and R40 roof so there will be major differences there. It also was on a very steep hill, with early morning solar blocked by the mountain side we were on and late afternoon sun blocked by the mountain across the valley to the west. While we lived there we would go on walks and have had our eye on the property we now own since 2015, it finally came up for sale this year and we bought it immediately.
    The problem with larger batteries:
    The ones you see on Amazon an usually Lithium Phosphate, they have a relatively short lifespan so you have to start doubling and tripling the cost as you prorate.
    If you look at the big solar guys they are tending toward heat storage in megawatts and using the phase change benefits of sodium and other molten salts (which aren't practical for my tiny project). But they are (currently) tending toward heat storage vs. electrical storage. In fact, I am really tempted to buy one ET 30 tube panel with the controller and run some real world tests in a 55 gallon barrel here at the current house just to get some real-world numbers. The only reason I haven't done it already is because I am having a terrible time hiring home care help for my wife so I can have the time to do it.
    Quick questions:

    Should I start another thread that is about my whole project?

    If you were buying triple pane windows in the next 6-8 months which brand would you buy? I have picked one that I have researched, but don't want to bias any answers (by saying which one), to this question in the hope there is something better out there, in either quality and/or performance.

  44. Expert Member
    NICK KEENAN | | #97

    "Should I start another thread that is about my whole project?"


    "If you were buying triple pane windows in the next 6-8 months which brand would you buy? I have picked one that I have researched, but don't want to bias any answers (by saying which one), to this question in the hope there is something better out there, in either quality and/or performance."
    This is worthy of its own thread as well.

    1. Expert Member
      NICK KEENAN | | #99

      I would say there are two separate but related questions:
      1. Is it better to take energy off the roof as hot water or as electricity?

      2. It is better to store energy as electricity in a battery, or hot water in a tank?

      Note that the answer to these questions is largely independent of scale.

      1. wastl | | #100

        are the alternatives not rather
        - thermal solar + PV or
        - only PV ??

        off grid - I do not see that fly without some PV+battery setup

        On-grid -> you could argue about what you have mentioned, off-grid the answer might be different (diminishing return, question of scale, ..)

  45. PAUL KUENN | | #98

    "you say you're average 21.9 kWhr/day of electricity usage."
    That is what the Thermolec heating unit uses per month in cold winters. The nasty monopoly of WE Energies doesn't allow using solar directly into house load center (breaker box). All solar pv power goes out on a separate meter (which we pay double for) and power we use is on another meter. They want total control even though our neighbor uses all of our power as electrons go...
    Our solar output with the old 1.9KW grid system is about 3KW per day in the dark of winter. The newer battery bank 4.9KW solar system averages 7KW in our cloudy winters. The best news is that I got rid of the huge lead acid batteries and have a new 12KW lithium iron phosphate battery:-)

  46. onslow | | #101


    Not sure how Duda is assigning the elevation difference, it would make some sense for solar purposes. 3300 less feet of atmospheric haze would certainly help the output numbers. Still doesn't account for the weather differences between 5300' and 8700'.

    If you have lived just down the hill a bit, then you already know true conditions over time. Maybe things here on the west slope are more chill. The zone maps do put the Pueblo area in CZ4, but I would suspect that only holds for the valley floor. If you really have mild winters at 8700' the peak loads will be more manageable. I was noting your reasonably close proximity to Monarch.

    Re windows. I went with Alpen six years ago. I looked into Zola and Intus and a private custom brand along with the big three box brands. The euros were at the time crazy expensive, long lead times, and the costs and risk of damage in transit too high to consider. Do be aware of the subtle differences of how euro windows are rated. The big three had very mediocre numbers for their windows. Pella was still doing interior storms which I grew up with and did not like. Marvin wouldn't ship with argon in the windows over 4800', I forget why Anderson got moved off the list. I seem to recall questioning the durability. I had used a number of their windows on projects before I moved. Cost effective was their virtue. Oh, and Sierra Pacific wouldn't ship with argon either.

    I was well aware of the film issues when Hurd first tried to push high performance windows way back in the 80-90's. I even saw the mess Serious Windows made of things. I may sound like a fan-boy, but I plan on using them in the current build unless something explodes. They have been using the new thin glass interior panels for over a year now, so I am intrigued. They still offer the film style and mine have been just fine in the brutal sun here. The air tightness is excellent. The hardware on casement has multiple locking points. The pultrusion filberglass frames do seem to play well with the glass expansion. AND I get to keep the argon fill. Krypton optional.

    I can't assess any of the euro-like offerings from out east. I would note that 2500 miles of trucking will be pricey and support may be a bit dicey.

  47. dlfdk | | #102

    For projects like this I always find it valuable to look at historical data in addition to means and standard deviations. An average may hide the extremes that are critical to factor in as well. And although past performance of an offgrid setup isn't a guarantee for future performance, it is nice to know that it would at least have done well then.

    There are some excellent datasets out there, courtesy of your very own government in fact. I've attached a full dataset covering the years from 2005 up to and including 2015 for the location of Canon City.

    The original dataset is hourly, which can get a bit unwieldy in the typical spreadsheet, so I've remapped it to daily, with insolation summed and the temperature averaged over each 24 hour period. That's 4017 data points for the period (two leap years). The insolation is based on global irradiance at a 62 degree angle, which I gather is the ideal for December at that location. The units are watt-hours and celsius.

    And since I already had the data, curiosity got the better of me, and I cooked up a super simple calculator - mostly for my own amusement, but it may be of interest to you, so here's the link. It's all in metric, which I realize is a bit of a nuisance for those most comfortable with customary units.

    What I find most interesting, when looking at the simple model, is how little energy storage actually affects the overall performance. A ten fold increase in storage, even with the unrealistic ideals used, have markedly little impact on the amount of backup heat needed. It wouldn't surprise me if you could smooth out the diurnal swings primarily with the building mass, especially if you choose insulation with a higher heat capacity. I'm thinking cellulose or wood fiber.

    There's links to the sources and datasets used at the website.

    1. mhenson | | #106

      Thank you dlfdk,
      that calculator is really handy. We are actually closer to Buena Vista's climate but with much more sun. So I will run those numbers. Part of the problem is Canon City includes a LOT of Fremont County and varies between about 5000ft and 10,000ft elevation. Our property is at 8,700ft.

      1. dlfdk | | #109

        I did gather from an earlier reply of yours that Canon City wouldn’t be the best proxy for your location, but I didn’t try to suss out a more accurate one due to privacy concerns.

        I have since updated the calculator to allow custom locations, so now it's possible to pick a better proxy. The data is fetched through the browser from the servers at PVGIS, so their full dataset is available, which covers most of the globe. Pretty fun to pick a spot in the Sahara!

        1. mhenson | | #110

          Thank you

    2. Jon_R | | #107

      It's a nice calculator. Note that it doesn't allow much storage - a max of about 1.4 cold days worth in this case. Would be better if it allowed more.

      It shows that when you have enough panels to provide 87-90% of the heat needed, storage works well to improve this to 99-100%. But no doubt that chasing the last few % isn't many kWh in absolute terms. Occasionally burning a little propane significantly reduces solar system cost.

  48. frontrange | | #103

    Another thing to consider to get the most heat out of thermal storage is to lower the temp of the water going to the radiant heater (aka the concrete floor in your case) at design condition. The two methods to do this are to increase the thermal conductivity of the heater (can't really do that with uncovered concrete but covering the concrete would make it worse) or increase the surface area of the heater (again, can't really add sf to the floor). But you could add radiant walls or ceilings, which could greatly increase the area and will be much faster in temp response than a concrete floor. You'd have to run the numbers to see how much temp reduction you'd get, but for example if the design condition used 90F water instead of 110F water, that's basically a 30% increase in heat storage (assuming 180F max storage temp).

    Probably makes more sense to just add more water storage but thought I'd suggest it.

  49. jj1 | | #104

    Hi Matt: Building a solar heating system which has sufficient BTU heating capacity for the coldest winter days, which have the fewest hours of sunlight, in a cold alpine Colorado mountain climate is very challenging.
    You probably would have to greatly expand the cost and capacity of the system to meet the heating BTU demands of the several days (or perhaps weeks) when the temperatures are lowest and the solar insolation is lowest. Periodic consecutive cloudy cold winter days are inevitable.
    Such a system would be significantly oversized and significantly more costly relative to the BTU heating load and BTU collection needed for "most" winter days.
    Some back of the envelope guesstimates: start with #95 Roger Berry's guestimate of possible 492K BTUs per day heating load at 0F design temperature or 20.5K BTUs heating load per hour. A possible 75K-100K BTUs of load could be covered via a combination of passive solar gain through south facing windows on a sunny day and extra attention to air sealing and thermal bridging or moveable overnight window insulation.
    Gary Reysa's DIY Solar Shed cost $4200 of materials and produces 162K BTUs stored in its 500 gallon tank per sunny winter day:
    Gary Reysa's DIY Solar Space and Water Heating system cost $2100 of materials and produces 65K BTUs stored in its 170 gallon tank per sunny winter day, of which 12.5K BTUs provide 30 gallons of domestic hot water with 52.5K BTUs left over for space heating:
    Total for both DIY systems is 215K typical sunny day heating BTUs.
    If you added a DIY build of a second of these Solar Shed projects then total sunny day heating capacity could be approx 375K heating BTUs collected and stored. There would be 500+ SF of collectors which have to be sited at a latitude appropriate tilt and maintained. (Of course, evacuated tube collectors alternatively could be utilized albeit at higher cost).
    Some savings could accrue via raising the tank storage capacity using a combination of: 1)make the 500 gallon tank larger 2)use a higher typical peak daily tank temperature of 150F-160F rather than the 125F Gary Reysa used and 3)possibly add sufficient micro encapsulated Phase Change Material with a melt point in the 125F range, to the storage tank.
    Nonetheless, all three of these DIY builds combined likely would produce insufficient BTUs to hold a 70F house temperature during the most challenging cold/dark winter conditions which are inevitable. There simply are not enough sunlight BTUs available to collect during these extreme periodic time periods.
    A backup heating system using wood or propane (or both) is needed for off grid.
    It might be simpler and more cost effective, use less space with less maintenance, to utilize the backup systems during these most difficult time periods, and instead size the solar heating system(s) to provide, say, "most" (perhaps 75% range?) of the annual heating load?
    Just an idea.

  50. mhenson | | #105

    Thank you Jan,
    I am considering all of these options, the insulation we will be using (as mentioned previously) is cellulose. I am trying to get good numbers to balance all of these variables to what is most efficient, simple, and cost effective. Can you provide me a link to the "micro encapsulated PCM with a melting point of 125F. If not too expensive that would provide a KISS solution to reducing the tank size.

  51. jj1 | | #108
  52. mhenson | | #111

    In the next couple weeks I am going to buy a Duda 30 tube panel with controller and accessories. Since winter is close it will be a very good time to test if these will work. I will probably use a glycol solution and run it in a loop to a 55 gallon barrel inside the garage. Then track flow rate, water temp inlet and outlet, ambient inside and outside the garage and get some accurate BTU/kWh numbers for this panel.

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