GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Skipping radiant floor heat in a passive house?

Jenz Yoder | Posted in PassivHaus on

I’m building a house with (conservative numbers) R 60 walls and R90 ceiling. The windows are in-swing casements with double seals. The floor is going to be stained concrete on 2″ insulation. I had 2 consultants tell me that I will not need radiant floor heat it will be too much. We will have a whole house air circulation system and a gas fire place. I am worried about not putting in the pipes in the floor and then being wrong. I am considering just putting in the heated floor under the rooms away from the fire place. I could add a heater to my whole house ventilation system if needed. Any thoughts. Oh and I live in heating zone 4.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Charlie Sullivan | | #1

    There are two questions. One is the size of your heating system and the other is whether radiant floors are a good match to your need.

    Regardless of what you heat with, you'll need to figure out the size. So either do a heat load calculation or hire someone to do that.

    If you want radiant floors because you think it will be nice to walk on a warm surface in the winter, you are out of luck. If you make the floors that warm, the house will get way too hot and you'll need all the windows open.

    I guess you could put radiant tubing in a small area, and heat that area to a comfortably warm temperature without overheating the whole house.

    It is hard to estimate how much temperature difference there will be at different distances from your single-point heat source. WIth your thick walls, not much, but some. That might be a good thing--you might want a different temperature if you are moving around doing something vs. sitting still by the fire.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Jenz,
    You hired some smart consultants. You should listen to them.

    Your house doesn't need any in-floor radiant tubing.

    For more information on this issue, see All About Radiant Floors.

  3. User avatar
    Dana Dorsett | | #3

    The gas fireplace is likely to roast you out of the house at your R-values, even when it's below 20F.

    The 2" of foam under the slab is a bit thin in proportion to your wall R even without radiant heat, and thinner still if you intend to use the slab as the radiator.

    Your wall-R is kinda "crazy high", and even half that much might not be financially rational against energy use costs. (You're basically betting that future energy costs are something like 2x the lifecycle cost of small scale PV solar at 2015 installed pricing.) In a US climate zone 4 location it's possible to build a Net-Zero-Energy house with wall performance of about R30 (with all thermal bridging factored in) or even R25 if you're VERY careful.

    For a quick & dirty starting point for where you need to analyze the financials very carefully to go much further, see Table 2, p10 of BA-1005:

    http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view (the link to the PDF is in the lower right corner)

    If you look at the row for Zone 4 you'll see they are suggesting R25 walls, and R60 attic, with R7.5 under the slab. At 2" thickness EPS foam is about R8-R8.4, so you got at least THAT part of it in the range of their analysis, but you're at more than 2x the wall-R, and 1.5x the attic-R.

    Spending the "extra" insulation money on 1-2 small air source heat pumps and rooftop photovoltaics ( PV) is probably a more financially rational investment. In 20 years when they are nearing end of life the replacement equipment or PV will be both cheaper and higher efficiency than they are currently. The decades long "learning curve" on the installed cost of small scale PV is about 20-25% cost reduction every time the installation volumes double, and the volumes are doubling every 18-20 months over the past few years and accelerating as it becomes ever more price-competitive with other forms of grid power. In 20 years it will likely be the very cheapest energy source, bar none. So even if the "extra" R30 -R35 of wall-R is cheap, it's still not necessarily every going to cheap enough on a lifecycle basis. In an R30-walled home occupant behavior makes a much larger difference in energy use than another R30 of insulation ever could. (Half of nearly-nothing is even less than nearly-nothing.)

  4. Jenz Yoder | | #4

    While I love your answer Dana in my case the cost of insulation does not apply. I have all the insulation i need to do the walls of my house with 9" of poly iso insulation for the cost of 1400.00. The house is 2300 square feet. I was blessed enough to find 3" x 4' x8' sheets of rmax brand taken off a large commercial building roof. I am doing double stud walls with a complete thermal break between them. Kinda of like doing SIPS. I agree comparing to the walls the under slab insulation is weak. This insulation is over a 5,5 inch slab of concrete that is designed for unstable soils.that makes my floor 11" thick so I decided to to stay with 2". Also in the center of the house is a 8' x 13 concrete vault/storm shelter with 8" thick walls and roof. That should help keep temperatures even. The property is already off grid with pv and wind. Our elevation ensures that no matter what the day time high temperatures are we always dip below 70 degrees at night. By using a whole house ventilation system we can cool the house at night and should need very little cooling. We are designing the garage entrance like an air lock to help with that.

  5. Richard McGrath | | #5

    Your query is very good indeed . Not sure how smart the consultants are . seems they may just be sheep following the more well known names and their opinions .
    A house like yours could very well require temps in the sub 90* range to heat at a design of -20* , which would mean they would be lower yet for greater than 85% of the season . What nobody ever mentions is that you CAN also use that same tubing to cool , at least handle the sensible load with warmer mediums requiring less energy . And , NO , they will not condense since they are a bit higher in temp than the dewpoint . Balanced ventilation and handling latent moisture loads can easily be handled by DOAS system . Which you will require anyway . Zoning becomes important , well it always was , in a house like you describe . Unfortunately it was ignored , this in turn caused at least some of the problems mentioned in Martin's epic " All about Radiant floors " .
    Warm floors was probably a poor choice of words during early marketing but let's face it , what we really want is a comfortable home . Please see the following discussion that is active at present . Pay close attention to the language used in the BSC paper .

    https://www.linkedin.com/grp/post/3898754-6042147576575184900?trk=groups-post-b-all-cmnts

  6. User avatar
    Dana Dorsett | | #6

    But Richard, his 99% outside design temp is likely to be around +20F, not -20F. He's in US climate zone 4 (probably 4B or 4C based on the summer nighttime temps), not zone 7. That's half the heat load your thinking of.

    And, hydronic heating & air conditioning in an off-grid home? Really?

    It's not worth the extra battery costs (unless a deal on batteries as good or better than the reclaimed polyiso deal comes along.) The super-insulation and maybe a small wood stove is more likely to be the "right" heating solution.

    When using reclaimed polyiso the cost is sometimes cheap enough that otherwise ridiculous R values become affordable. Typical 2lb roofing iso runs about R5.5/inch so that's really more like R50 than R60 walls, but still overkill, and potentially awkward to build, but it's probably worth taking the time to do it right. To retain the benefit of the super-R pay close attention to minimizing structural bridging elements between the pair of studwalls. It doesn't take a huge cross section of wood or steel to exceed the heat moving through few square feet of R50.

    I'd love to see pictures of this off-grid house as it goes up!

  7. Jenz Yoder | | #7

    The house is a rectangle to help with framing the walls in a way to have a complete thermal break except for openings The main reason I''m interested in heating and cooling in floor is we live in a high wind zone and the dump load from our wind turbines heats water which could be run through the floor. I also have a lot of acreage and can run a trench to put pipes under ground for cooling. Both of these only involve running a circulation pump. I am a DIY builder with previous exsperiance in all phases of construction. Since I have no labor costs I can afford to do things differently. I installed my own PV system which has been successfully running our living quarters in the barn for 6 months. We average wind here year round that l gives night charging capability. The cost of running electric to our house would be 20000.00 so we decided to go off grid and so far have only spent 14000.00

  8. Richard McGrath | | #8

    The right choice of a water heater and a large enough tank are key here . First off , we should address the health concerns with storing water that are often missed . Hot water should be stored at no less than 140* and mixed down to =<120* . The house we are discussing with it's elevated levels of insulation (would love to hear ACH estimate) allows the opportunity to use ODR and the stored water in the tank without compromising DHW and almost no energy penalty . Since he has an excess of electricity during windy times this is an exceptional case .

    ECM pumps would be adequate for everything having to do with heating and cooling for this home . So , in answer to your question Dana , Yes , for an off grid home . The numbers I have based my comments on come from a super tight house which I modeled that I can move anywhere in the world to see how it will perform .

    Again , some commenting have not heard or understood the Ops question or desire . He wants radiant systems and is looking for verification of the advice he has been given by 2 consultants whose knowledge may or may not be adequate to review the situation and DO THE MATH . GBA was quite possibly mentioned to him ( maybe not) by one or both of these individuals to validate their opinions . Most people that want radiant heat are prepared to pay a bit more for it . Boilers are not required anymore and an appliance that resides in every first world country home can be utilized , this alone brings the first cost down exponentially . It also offers the opportunity to eliminate a big PITA called standby losses . Hell , in summer during cooling season he could even run the water that has removed heat from the house through a smaller pre heat tank prior to his water heater which could allow for smaller equipment or shorter loops ( GHX) . The COPs related to this type of stuff are far greater than anything ugly you could hang on a wall that uses outdoor equipment and refrigerant . Bottom line is some people want something better . From a building science standpoint , this is better . Maybe you could explain how using <80* water to heat at exaggerated designs (hedge) , employing ODR , 70-75* surface temps and handling the sensible load with water that is 3* higher than dewpoint while allowing his balanced ventilation system to deal with latent is a bad thing and won't work in an off grid home ?

    It is my opinion that an off grid house should have the luxury of several choices to heat and cool . WATER can be heated and cooled in many ways , how many anyone would like to install to hedge against any number of things is his choice .

    Jenz ,
    These are my opinions and there are plenty of folks who can help you do this properly . Your location would be very helpful and a room by room heat loss and blower door target would be good . My suggestion to you would be to visit healthyheating and heating help . The folks there are more than willing to explore more than one option and offer the BEST ADVICE on heating and cooling systems based on more than just their opinion and first cost . The discussion I linked to above offers BSC's opinion on what they deem best , then states that it may not be necessary in a super tight home . Robert's response is right on the money I might add .
    You should certainly consider radiant a very good choice and since you have stated that there is room for ground loops and some other opportunities . You should also consider getting the advice of people whom have done this successfully and not failed because they lack the ability to determine that scavenging EVERY BTU has it's penalties .

    Wish you luck in your endeavour and should you want to speak with me personally feel free to do so . Connect with me on linked in maybe or visit heatinghelp.com , I am very active there .

  9. D Dorsett | | #9

    I'm not saying off-grid hydronic heating can't work or shouldn't be done, only that it's an expensive solution for a small/very-small problem. ( It's sorta like using a smart-phone as a fly swatter. :-) )

  10. Richard McGrath | | #10

    Would that be similar to installing a 3 ton mini split in a 2 ton house because at cold temps we know it only provides about 65 - 70 % rated output ? Dana , I am curious as to how much energy those use , could you tell me ? Maybe your answer could be based on an entire 2500 hour season in total KWh consumption .

  11. Richard McGrath | | #11

    Charlie ,

    Radiant will not overheat a home of any type .. When the surface temps reach the temp they need to be to satisfy the stat the zone shuts down , no more flow , no more heat . If we look at it in another way , During the day when there is solar heat gain on the South side and the temp is maybe 20* the floor's heat will maintain an MRT in that room and the radiant will remain off , should the radiant come on for any reason the fluid flowing through the floor in my sample house would be 72* . Not much chance that 72* fluid meant to make a surface temp of 70.8* is gonna transfer any heat to a slab that is likely already higher than that . No?
    That is why zoning is so important in this type of house . Just like you have said and I have verified , the floor temps will not be a far departure from ambient If it only requires 72* fluid to reach 70.8* surface temp the chance of that room overheating is nill . As a matter of fact that fluid will carry some of the heat away and deposit it elsewhere where it will do some good . OOps , another myth debunked .

    The HRv certainly must be a zone . With temps in this house never required to be above 80* most any water heater will do the trick dependent on what type . When you store at 1408 to eliminate any nasty stuff (legionella) and mix down for DHW and also use an outdoor reset mixing valve you can heat the house for a long time while holding the source out from firing . One more point for water . The minisplit cannot provide anything unless it is consuming gobs more energy .

  12. Richard McGrath | | #12

    There is another opportunity I forgot to mention . He is going to have a fireplace . A Tulikivi W10 can also provide heat to that heater or a storage tank . This would enable this man to go to sleep at night and let that fire go out and use no more fuel sav3e for a tiny bit of electric to heat the house very nicely till the morning .

  13. User avatar GBA Editor
    Martin Holladay | | #13

    I have lived in an off-grid house for 40 years.

    With a big enough battery system and a large enough wind turbine, you can generate as much electricity as your bank account allows (except, of course, on those dark windless days during the winter).

    I'm not in favor of fireplaces or complicated heating systems that depend on electricity for an off-grid house. Keep it simple.

    An ordinary wood stove works better than a fireplace, and it's much cheaper than a $14,000 Tulikivi.

    I know of several off-grid homeowners who installed pump-based hydronic heating systems and abandoned them because they didn't like the way the pumps depleted their batteries.

    But hey -- it's your house. The simplest way to keep your house warm when you aren't there to feed the wood stove is with one or more propane space heaters with through-the-wall vents.

  14. User avatar
    Dana Dorsett | | #14

    Richard- I wouldn't recommend using mini-splits to heat & cool an off grid house either. It would be a worse load than hydronic solar-thermal heating solutions. Batteries cost real money, and multi-kw loads are best scheduled rather than run automatically in background to keep the local 1-house micro-grid from being abused. Everything has to be sized for the peak draws with some margin, as well as the total energy storage capacity, so keeping the peaks low/very-low and the total energy that needs to be stored in batteries low is key to making it affordble.

    The notion that you'd ever use "...3 ton mini split in a 2 ton house because at cold temps we know it only provides about 65 - 70 % rated output..." is utter bunk. The tonnage ratings are cooling BTU/hr, and the heating output of a cold-climate mini-split is typically higher at +5F than the rated cooling BTU/hr. A 3/4 ton Fujistu 9RLS3H can deliver something like 15,000 BTU/hr @ +5F. Sizing a mini-split for heating is best based on the calculated heat load and the extended temperature capacity tables for the unit at the 99% outside design temp, not by it's rated cooling tonnage.

    The efficiency of cold climate mini-splits fall off to COPs in 2s or lower at temps below +10F, so they're not really great solutions for zone 7 climates. But in a zone 4 climate the better ones will avearge in the mid to high 3s (in rare instances, 4), and have WAY more heating capacity than the rated cooling tonnage.

    Calculating the home's mid-winter heating load on a weekly basis relative to the best wind survey data you can find (or make) would be important before relying on a thermal power dump / buffer tank for the excess wind & PV production. It's a rare house/site that would produce sufficient excess turbine power to heat the house, even the gnat-sized heat loads of an R50 house in climate zone 5.

    I read it was a gas fireplace, not a wood-burner, and I recommended against it. A (also pricey) high mass unit like a Tulikivi might do just fine for heating and heat the domestic hot water too, but maybe not. The domestic hot water is usually the largest thermal energy use in any high-R house, and the heating season will be relatively brief in a well designed high-R home in zone 4. Solar thermal hot water may make sense, especially if the thermal capacity the solar can be combined with the power dump for the wind turbine.

  15. Richard McGrath | | #15

    Dana ,
    As usual , well stated . I would never even bring up solar thermal for fear of another of Martin's epic papers being brought up . Can't fight battles on too many fronts .

    Martin ,

    What kind of pumps did your associates use that depleted batteries so quickly ? They certainly were not DC ECM circs were they ? You continue to mention these complicated heating systems . Maybe that's the problem that many of your reference projects experienced . Too many people believing they were smarter than they actually were using too many components trying to scavenge too much energy that was not really available . Glycol , pressurized solar thermal panels never were my choice . Just as storage is the biggest factor in electric production so goes heating and cooling . I must ask your opinion on the difference and attractiveness of site derived vs site generated ?

    I have no idea what your home has for off grid features but am certainly interested . Maybe you could message me to discuss . You keep stating that you have lived off grid for 40 years , what bearing has that on our discussions other than you possibly have alot of 40 year old stuff and due to your philosophy you and yours make concessions during certain times of the year ? Everyone does not have to make concessions and may even see the benefit of being able to produce or harvest energy by several different means . Is that not what off grid is about , never being dependent or better yet , being self reliant ?

    It amazes me that a man that is as recognized as yourself has only the inclination to promote one type of alternative and damn everything else . How do you stand to benefit from such behavior ? It's a good thing you live at elevation martin and will probably never witness the power of water , it is in fact my friend the most powerful single force on this Earth . It stores and releases energy in a far superior manner than anything else . At the end of the day it is also the safest , go figure .

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Richard,
    The main reasons that off-grid homes have a different approach to choosing electrical appliances are (a) batteries are expensive, and (b) most off-grid hopes rely on PV, and there isn't much PV power available from Nov. 1 to Feb. 10 in most of the northern half of the U.S.

    Running a gasoline-powered generator to power a house and charge batteries is expensive.

    As usual, high-income families with big bank accounts can pretty much do whatever they want. That said, even wealthy families may not be satisfied with the usual solutions to the dark days of November and December. It's true that you can always buy a bigger battery bank, but those cells need to be topped up with distilled water, and maintenance is a pain. Moreover, you can always run your gasoline-powered generator for more hours per day, but the generator is loud; you have to change the oil; and no one likes hauling gasoline and fueling the beast.

    There are a constellation of factors affecting this discussion, of course, and the economics of off-grid systems is changing. As we all know, PV modules have gotten dramatically cheaper in recent years. (Although if the sun doesn't shine for two weeks, a big PV array doesn't help much.) And there are signs that, two to three years from now, batteries may become cheaper. Those factors will make off-grid homeowners' lives easier.

    It is possible for a smart engineer to design a hydronic system to use very little electricity, and I don't doubt that you can design such a system. The problem is that very few plumbers or heating contractors know how to do this. Moreover, designers of boilers, fuel pumps, and oil burners don't care about electrical use.

    Most hydronic systems have oversized and inefficient pumps. Is the problem solvable? Of course. But there are many opportunities for off-grid homeowners to be surprised and disappointed when they tell their heating contractor, "Install a hydronic heating system. I'll just run it off my batteries."

    A hydronic heating system is a one-off system designed (or not designed) by a local contractor or (in some cases) an engineer, and hand-assembled on site by a local contractor using parts purchased from a variety of manufacturers. There are a lot of opportunities to screw things up.

    In contrast, a propane space heater with a through-the-wall vent comes as an engineered system from a single manufacturer. It isn't particularly efficient, but it's simple. It is easy to predict this appliance's electrical draw. It is zero.

  17. D Dorsett | | #17

    Yabbut Martin... we LOVE highly efficient Rube Goldberg contraptions (especially when they work! :-) ).

    The design risks and power draw make hydronic off grid something more for the wet-head purists and energy-nerd hobbyists with the budget and risk tolerance than something to recommend for the typical off-grid house (as if there really is such a thing as "typical off-grid house". :-) )

    The fundamentals haven't changed. Assumptions & calculations of the total power reaped by single turbine small scale wind are usually dashed by grim reality once implemented, even in windy locations. PV is far more predictable, but not favorable for heating season uptake. Heating systems that require power are thus risky in any off-grid implementation.

  18. Charlie Sullivan | | #18

    Dana, I think "typical off-grid" homeowner and "energy-nerd hobbyist" might be closer to the same thing than you are allowing for ... and anyone who goes to the trouble to specify R-60 walls is likely to specify efficient pumps. Of course, in either case, getting competent local contractors to implement it right is difficult, and I certainly agree that packaged systems are what the general market needs. But that's not a reason to discourage energy-nerd hobbyists from doing their thing.

    Richard, my apologies: I wasn't clear. When I said that "You don't need much area to deliver enough heat to overheat the house" I didn't mean that a well designed system would actually overheat the house. I only used that wording to emphasize that the full floor area is way more than you need to adequately heat the house. There are advantages to using the full floor area, in that it allows using lower temperature water, and as you point out, the use of low temperature water makes controlling the system to avoid overheating easier. But there are advantages to using less than the full floor area too--you save money on tubing and also on either pumping power or manifolds. And you can now heat those floor areas to higher temperature, if you want to.

    When I see a twenty line post as a rebuttal to what I said, and yet I agree with every one of those 20 lines, that is a sure indication that I wasn't clear in the first case. I hope I have done better this time.

  19. Charlie Sullivan | | #19

    The fact that you are heating water as a dump load for a wind turbine changes the equation for me and makes it make more sense to include radiant floors. You don't need much area to deliver enough heat to overheat the house, so you could opt to put it in selectively, such that a few areas are sometimes warm enough to give a bare-foot comfort advantage, and at the same time save money over putting tubing throughout. [Edit: Using the full area works too--and I didn't mean to imply that it would overheat the house, if done right. Just that it's more capability than is needed.]

    Another way to use that hot water would be to put it through an air-to-water heat exchanger on the output of the HRV to heat the air being delivered through the house. But depending on how much hot water you have available that might not use it all.

  20. Jenz Yoder | | #20

    Well every body I've made my decision. After calculating the costs for the materials in the floor I decided that 700.00 is a small amount to risk on something I might not need. I am spreading the pipes apart more in the main areas and less in the bathrooms. I would rather have the pipes in the floor and not need them then wish I would have spent the 700.00 to install them. This house is an experiment so I will find out. The slab has a complete thermal break so it will be efficient to heat. I realize that due to normal costs my decision does not to apply to most but since I have the luxury of being my own labor it allows me to do things that are normally not cost effective. We have no Zoning or codes here so it up to me to do it right.

  21. Jenz Yoder | | #21

    Oh and as for circulation pumps I am currently researching dc pumps as well as a dc fan hrv. I live in the high plains of Texas so wood heat is not a great option since it must all be trucked in from a distance. With our current trends on wind and sun I should be going to bed every night with a large amount of hot water in a well insulated tank. I am hoping that a well insulated house with a warm slab will work for me.

  22. Richard McGrath | | #22

    Jenz,

    Make sure you use barrier pex in the floor . Not knowing your exact conditions I will say that farther spacing will require higher water temps to achieve the same thing . In an off grid situation this could be the difference between a failed experiment and a successful one . Watch out where you are sourcing your information , this too has created more problems than it has helped folks .
    Case in point , one internet company touts what they call an open direct system . Something like this is a bad idea in any house but in yours it would be disastrous . I am available to assist you with any questions or ideas .

  23. Michael Ginsburg | | #23

    Jenz Yoder- I wish I had come on board this debate/discussion a couple weeks ago because of all the misinformation, erroneous predictions, etc., etc.
    Most, if not all, the opining and consulting you have endured with your realistic questions have been answered from those who have not, in fact, done what you are doing; those who only rely on conjecture. They have not been in the 'trenches" as your are doing and have not "been there, done that" in the real world where the proverbial rubber meets the road. I have been living in a passive house quality, proven home with radiant floor heating AND radiant floor cooling in the desert southwest, Tucson, AZ for four years now. One that was tested and monitored by the DOE Build America Research Program.
    I can say without hesitation that radiant floor heating in a passive house quality or qualified home is by far and away the best heating, and cooling system you can have for this "ice chest" like home. What these 'experts' do not realize is that in this kind of "ice chest" like home, the insulated and thermally isolated 5" thick concrete slab equates to the "blue ice" that one puts in the ice chest except it is "plugged in" to a typical wall thermostat and corresponding heat/cool source for the water. By keeping the surface temp of the slab at 75 degrees (or whatever you choose), the entire living space both horizontally and vertically stays whatever floor surface temp you have chosen. The entire volume of space is the same temperature within degree or so. The enormous amount of thermal mass that the concrete floor contains trumps anything else, period. And, by the way, the time for "recharging" the floor is during the day when the sun shines and having a modest size PV system will be all that's needed for heating. No electric storage needed. And because of the superior quality of your thermal envelope your heat loss will be immaterial and there will be no lag time between heating days.

  24. Richard McGrath | | #24

    Thank You Michael Ginsburg . You must live in a house designed by someone who had a clue about how to properly zone and install such a system taking the truly important factors into consideration . Radiant cooling , you say . They say that cannot be done , but you and I know better . Could you tell us some more about your home ?

  25. Charlie Sullivan | | #25

    Radiant cooling in a dry climate is much easier than in a humid climate.

  26. Richard McGrath | | #26

    I would point out that no matter what equipment you are using Charlie , there are few that do a good job with latent . Mold is a growing problem since high efficiency equipment is being installed in a different type of building by guys who don't know what they are doing .and count on equipment to do their work for them . There is alot of radiant cooling in very humid areas that works well . 2 examples are The Infosys building in Hyderabad India and the International Airport in Thailand . One place is wet all year and the other is about an even split of moist and dry . We could agree both are HOT !

  27. David Butler | | #27

    Rich send me this link... he knows I'm among those who advocate against mixing thermal mass with heat distribution, especially in low load homes.

    Not only does a radiant slab waste energy to the ground, but response times are much longer. This can be an issue in the swing seasons, mostly due to glazing loads. In a nutshell, long response times cause energy waste when you have a rapidly changing load profile (e.g., heat load in AM, no heat load in the afternoon, or even a PM cooling load) and can even lead to comfort issues. All of this can easily be overcome by decoupling heat distribution from the building's mass. In particular, radiant with low mass emitters is a better solution for low load homes in my opinion.

    To Michael G's point... no doubt a well designed radiant system can be both comfortable and efficient. There are ways to mitigate swing-season issues, especially in homes not specifically designed to optimize passive solar gain in cold weather. So it's not my intent to dis radiant... even radiant floors. But it's not how I would have done it.

    My biggest problem with radiant in general is that it costs so much more than forced air (although this may not be pertinent to Jenz's situation because (a) he's off-grid, and (b) he puts no value on his labor). And to Charlie and Martin's point, finding qualified hydronics contractors is indeed a real challenge in many markets. But it's always easier to find good forced air contractors than hydronics contractors.

    Look, I realize most forced air systems are pretty awful. But it's incorrect to suggest that forced air is somehow inherently less comfortable than radiant in low load homes. The better the envelope, the less the "radiant is more comfortable" argument holds up since thermal variations and MRT's naturally get better when you build an igloo.

  28. David Butler | | #28

    @Rich wrote:
    "...you CAN also use that same tubing to cool... latent moisture loads can easily be handled by DOAS system"

    This may be true, but it's not rooted in reality. By adding chilled water, you've now excluded most residential hydronics contractors. Chilled water is a whole 'nother skill set . Also, a DOAS must have a substantial bypass mode if it's the home's only latent capacity. If you just stick a DH coil in an outside air duct, it won't remove internally generated moisture. Also, you gotta add a reheat coil to handle DH calls at part load conditions. So now you're talking about a complicated built-up system. The problem is that the commercial crews who know how to do this usually won't touch residential projects. They're looking for mechanical budgets well into the six figures. And if you're lucky enough to find one, be prepared to open your wallet.

    All of that said, in an off-grid home, everything changes. I won't repeat all the great points made by Dana, but I would add that thermal storage and thermal mass is your friend in an off-grid home. Conserving BTU's and kWh's become much less important than managing loads and limiting peak coincident electrical loads, and providing for backup when the sun (or wind) isn't there for you. Gensets are mandatory for all off-grid homes, but as others have said, they're noisy and expensive to operate, so you want to minimize their use.

    I'm currently designing the mechanicals for an off-grid home near Phoenix (e.g., cooling loads dwarf heating loads). The design objective is to support 3 days of peak cooling with minimal solar insolation, as can happen during monsoon. In that case, implementing a large thermal battery (5,000 gallons of chilled water) served to reduce power system costs by such a large amount that the additional cost/complexity of the wet mechanicals was way more than justified. However, even in a metro area as large as Phoenix, one of my most difficult challenges was finding contractors who were both qualified and interested in looking at the project. C'est la vie.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |