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Is there a benefit to having a radiant floor heating system in a passive solar home?

user-6906608 | Posted in Mechanicals on

Hello. My wife and I are having a passive home built next summer. The house will be located in the Drake, Colorado, at about 7000 feet. climate zone 5. The home will have a slab on grade foundation. The home construction method will be a modified post and beam structure with straw bale infill(non-load bearing). The home will be one level. There will be no furnace and no AC. Heating would be via a wood stove insert into a fireplace. The goal is for a tight envelope. Walls will be approximately R-40 with the straw bale infill(this is covered with a welded wire material and then hydraulic lime plaster applied to both sides) and attic probably at least R-50? The roof will be a standing seam metal roof. We plan on having a 6-8 KW grid-tied solar panel system. There is no natural gas available. Propane is an option but my wife and I don’t like the cost, so our plan was to use the electricity.

My architect has recommended a radiant flooring heating system and solar water heating system
As this house will have a very low heat loss( 0.1 BTU’s hr/sqf or maybe less) it would not take much energy to maintain the temperature. During the winter months where the house will benefit from the solar gain, the concrete will already be absorbing that energy, so why would more heat need to be added, or is this system only laid in the areas outside of the direct gain areas, such as the bedrooms, bathrooms, etc.? I’m sure I am missing something here, but I need to understand this better. I have been doing my research and keep running into varying opinions on this. I understand there can be a benefit for cooling from the hydronic system during the summer months. I have read that I could have overheating problems with a combined passive solar/hydronic heating floor system due to slow temp changes in the slab? The other issue is cost, which I estimated to be at least 10K for the hydronic about 8k for a solar water heating system. Will I actually save at all?

The solar water heater systems sounds good in theory, but my concerns are risk of legionella in water that is <140. It’s my understanding is that it takes at least this temperature to kill the bacteria. Chlorine can also kill the bugs, but this will be well water so no chlorine. I have looked at Radintec’s website which is a sell direct to consumer company. They promote the Polaris water heater as a combined domestic hot water and for hydronic system vs. using a typical boiler system, stating the Radintec’s is more efficient and less costly. I would not be able to use that though, as i prefer not to use propane. So I would have to an electric water heater instead. I understand the solar water heating system preheats the water( do you really get your money back?) and then if needed can be heated further by electric. I also have read that the hydronic floor can be as a storage medium instead of a storage tank? Also of note my electric rates are fairly low at about 9 cents kw/hr. Thanks for any help. Peter

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  1. GBA Editor
    Martin Holladay | | #1

    Lots of issues here. When you talk about a "passive house," are you talking about a passive solar house, or a house that attempts to meet the Passive House standard (either the Passivhaus standard from Germany or the PHIUS standard)?

    In a certified Passive House (or Passivhaus), a wood stove may not be the best choice. Providing combustion air to a wood stove in an extremely tight house can be tricky. For more information on this issues, see "All About Wood Stoves."

    Your guess that passive solar heating principles are incompatible with radiant floors (floors with embedded hydronic tubing) is -- with a few caveats and qualifications -- more or less correct. For more on this issue, see "All About Radiant Floors."

    Finally, solar thermal heating systems require so much expensive hardware, while delivering such small amounts of usable space heat, that an investment in a solar thermal space heating system is never cost-effective.

    For more information on this issue, see these articles:

    "Solar Hot Water"

    "Solar Thermal Is Really, Really Dead"

  2. user-6906608 | | #2

    hello, passive solar house is the plan

  3. GBA Editor
    Martin Holladay | | #3

    If you are talking about a passive solar house (rather than a certified Passive House), you may want to read this article: "Reassessing Passive Solar Design Principles."

  4. Expert Member
    Dana Dorsett | | #4

    In locations with large diurnal temperature swings (that would be pretty likely at 7000' in CO) using the slab for a heating radiator usually results in temperature overshoots. The slab has to be cool in the AM to soak up the direct solar gain in the morning, but warm enough to still heat the place at the daily low temp just before dawn. Hydronic heating using lower mass radiators can still work though.

    If you're putting solar PV on the house you might as well skip the propane altogether (except perhaps as a backup generator when forest fires or lightning strikes temporarily take out parts of the power distribution grid not sure how common that is in Drake.)

    A 3/4-ton cold climate mini-split should cost less than $4K all-in (if ductless, maybe as much as $6-7K if ducted) and probably cover your loads, and would be FAR more responsive to rapid changes in load than most hydronic systems (even low-mass radiation hydronics.) A ductless Fujitsu ASU/AOU9RLS3H can put out 11,000 BTU/hr @ -5F, but can also throttle back to 1,535 BTU/hr at that temp. The specs for their mini-ducted version are fairly similar (though only specified down to -5F). If that's too much overkill, a half-ton Mitsubishi MSZ/MUZ FH06NA delivers 6400 BTU/hr @ -13F, and can throttle back to 240BTU/hr (about the same heat output as a sleeping human) at that temp.

    These things are also capable of air conditioning, but in your climate / elevation/ location nighttime ventilation strategies usually work fine for keeping the house comfortably cool, as long as you don't go nuts on excessive glass area.

    I'm skeptical that the heat load to square foot ratio would be as low as 0.1 BTU/hr per square foot of conditioned space at the 99% outside design temperature, which in your case would be about -5F, only colder than that 1% of all hours in a year. (The 99% temperature bin in Ft. Collins at 5000' is +1F, Boulder's is 0F at 5300' of elevation. Knocking off about 3F per additional 1000' is a pretty good rule of thumb, with lots of exeptions, but not extreme exceptions.) The 0.1 BTU/hr per square foot might be the heating season or annual average heating/cooling energy use or something? A more realistic heat load ratio would likely be in the 6-7BTU/hr per square foot range if it's a super-tight house. So a half ton Mitsubishi might barely cover the heating load for a ~1000' house, a 3/4 ton Fujitsu a 2000' house.

    For reference, this 1935' certified PassiveHouse had a calculated design heat load of 5,890 BTU/hr @ +23F (about 30F warmer than your 99th percentile temperature bin). That's 3 BTU/hr per square foot of conditioned space. The load ratio at your house would probably come in about twice that.:

  5. Brian P | | #5

    How many square feet is the house and do you have a floor plan?

  6. Trevor Lambert | | #6

    Make sure you do some energy modelling before you get too far along in the process. Some of your numbers sound unrealistic. I am in a similar climate zone (in Canada, but geographically very close to the zone 5 region of the NE of US). Our house is way more insulated than what you specify, and probably more air tight than you plan to achieve, and our heat loss per hour is more like 3-4BTU/hr. 0.1BTU/hr is so low you wouldn't need a heating system at all, just two people sleeping or a handful of LED lights would heat the house.

    A wood burning appliance is hard to make work in a super insulated house. If you actually get the house as efficient as you want, you'll have a hard time finding an appliance that won't overheat it. Getting proper combustion air and ventilation balancing is also a challenge. Check out, they advise against direct air combustion stoves/fireplaces. And when the stove is not running, it's a constant energy drain, because even if you manage to get the supply air vent to seal, there's no way you're going to achieve that on the flue, and the chimney is a non-trivial thermal bridge. I tried to find a way to fit one into my house, simply for the ambience. After much deliberation, it just didn't make sense.

  7. user-6906608 | | #7

    my error on the BTU, should be 0.1 x 75 = 7.5 BTU !!

  8. Expert Member
    Dana Dorsett | | #8

    OK, that's 7.5 BTU/hr per square foot- a much more credible number.

    How many square feet?

    (Why not just share the design heat load rather than a ratio, which is meaningless without the denominator.)

    There is probably an optimal (and affordable) modulating mini-split solution that "plays nice" with passive solar gain and fast changes in heat load, but without the design temperature and heat load number it's hard to guess with any accuracy.

    BTW: The 11K max/1.5K min output numbers for the 9RLS3H is at -15F, not -5F (as mis-typed in my prior post.) It's capacity at -5F is about 13.5K max/2K min. So if your design temp is -5F, at 7.5 BTU/hr per square foot it's good for covering a house up to 1800 square feet. (They make 1 ton and 1.25 ton versions too, if you need more than the 3/4 tonner will put out.)

  9. user-6906608 | | #9

    Hello to all who have answered these questions. My original plan for a heat source was for a ductless mini split but during a meeting with our architect he suggested the radiant floor system. in regards to the wood stove insert I wanted this for the ascetics and the occasional fire. The plan was never to use it as a primary heat source. My architect is a LEED Accredited Professional, Sustainable Materials and Energy Consultant, so I'm not sure why he would't be more aware of these different opinions. In regards to the radiant floor heating, I was skeptical and once I looked into it more, I didn't really see the advantage, nor for solar water heater. The other issue in my mind with more complexity there is a greater risk for more problems, and more time spent dealing with those problems. simple is better.

  10. user-6906608 | | #10

    To Brian's question, the size of the home will be between 1700 and 1800 square feet.

  11. GBA Editor
    Martin Holladay | | #11

    You can imagine why we got confused. In your original post, you wrote "There will be no furnace and no AC. Heating would be via a wood stove insert into a fireplace."

    In your most recent post, your wrote, "In regards to the wood stove insert I wanted this for the ascetics and the occasional fire. The plan was never to use it as a primary heat source."

    Some advice:

    1. Consider abandoning the idea of a wood stove and abandoning the idea of radiant floor heat.

    2. Your original idea to heat with minisplits was a good one.

    3. Consider hiring a new architect (even though your architect is "a LEED Accredited Professional, Sustainable Materials and Energy Consultant").

  12. Expert Member
    Dana Dorsett | | #12

    It's pretty odd that an energy consultant is out of the loop on where cold climate mini-splits are going. Download a copy of the NEEP spreadsheet, which lists capacities (min & max) and coefficients of performance at various temperature for a large number of heat pumps (not just mini-splits), including at the lowest temperature specified by the manufacturer.

    Send the link (or the spreadsheet itself) to the consultant!

    It gets updated regularly (the last time was two weeks ago.)

    Vapor injection compressors used in cold climate heat pumps have come a long way since they were originally introduced, improving both capacity & efficiency at low temps. The larger US HVAC manufacturers have been experimenting with that type of technology, but product releases don't seem imminent, whereas some mini-split vendors are on second & third generation versions of cold climate heat pumps now.

    NEEA's heat pump water heater list might also be useful to keep on hand:

    At current installed pricing of rooftop PV, solar thermal systems for domestic hot water or space heating is all but dead when compared to system cost/maintenance/reliability of PV + heat pump water heaters. See:

    (Including the extensive comments)

    and the previous link (by Martin):

    At your heat load a pair of $50 1500W oil filled radiator type space heaters (about 10,000 BTU/hr total) could serve as back up if you went with a single mini-split and it failed for some reason in the dead of winter. That's cheaper and easier to deal with that a wood-burner, but it still needs the grid to work (unlike a wood burner.)

    If a ducted mini-split works better than a single head point-source solution, Fujitsu's xxRLFCD series mini-ducted units are pretty good, even if the capacity tables only go down to -5F. The 3/4 tonner is good for 14,000BTU/hr @ -5F outdoors, 70F indoors, the 1-ton is good for 15K which isn't a whole lot more. But the 1.5 tonner a solid 18,400 BTU/hr (sufficient excess margin to take you well into negative double-digits, even though the output isn't specified there.) See the capacity tables that start on p.15 (PDF pagination):

    They can be mounted vertically in a 5-8 square foot "utility closet", if that works better than under a ceiling in an accessible spot. Even though point source heating with ductless works pretty well in high-R houses, some inspectors get leery about the ability to heat every habitable room to 68F without heat distribution system (pipes or ducts). A 1.5 ton mini-ducted Fujitsu solution should still come in WELL under $10K, but more than $5K.

  13. Walter Ahlgrim | | #13

    My guess is you will not need a loan and have no government inspection for your project but understand the lack of a permanently installed heating system will make this home hard to market and almost impossible to finance when it does comes time to sell it.

    Lots of people from Colorado after a few winters decide a 60 day vacation in mid winter is a great idea. How will this house fair if left alone?

    Check with your insurance agent but it gets more difficult to get a policy for any building with solid fuel for heat.


  14. user-6906608 | | #14

    Dana, sorry for the confusion, I should have laid out the original plan first (mini split), but I felt vindicated that my original idea was good and just wanted to express that it was the architects idea, and not my original plan.
    I am a novice in this area as you may have already realized! There is no design heat load at this point, no house design yet.
    In regards to the mini splits, I actually mentioned this to the architect, but this is when he suggested the radiant floor heating, said AC was unneeded(which is probably right, but you get with the mini split - bonus in my book.
    I too am surprise the architect recommended the radiant floor and solar water heater, in fact I had emailed my concerns to him with both the day prior, and have not got a response, so decided to get opinions here, as the internet is often fraught with biased opinions, i.e. As you all appear to have extensive knowledge and experience, now imagine a novice such as myself, finds a architect who has been designing these homes(passive solar, straw bale infill) for years and is a LEED Accredited Professional, Sustainable Materials and Energy Consultant"). After reviewing the gallery of homes he has designed, the following is described of his home, "Jeff's own home designed and built in 1992. Featured in Mother Earth News for its passive solar design features sustainable materials. Doug Balcomb of the National Renewable Energy Laboratory once said that this home uses the most solar energy of any home in the Roaring Fork Valley" After reading this I suspect his focus is on the renewable aspect over cost. I can't imagine he could be unaware of the efficiency of a mini split, I just think he favors renewable energy . I appreciate everyone's expertise and time.

  15. user-6906608 | | #15

    Walter, thanks for the advice on the "insuring solid fuel for heat homes", I was unaware of that issue. In regards to the heating source, I am aware that there has to be a permanent heating source, as I would not even be able to a get a certificate of occupancy without. Originally my idea was for a heat pump after researching this, and seeing how efficient they now are. The radiant floor heating idea was the architects bright idea, that never seemed to make sense to me in passive solar home. This home will be partially financed

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