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Whole-House In-Floor Radiant Heating

Pawan Bajaj | Posted in Energy Efficiency and Durability on

Is whole house radiant floor heating effective in the real world?

Calling out to homeowners with radiant floor heating…

I am looking at constructing a new home in New England, specifically the metro Boston area. A large house approx 4,500 sq ft on 3 floors, I would like to use radiant floor heating throughout, mainly for comfort but secondarily for hopefully lower energy bills.

The house would be well insulated and my concern is that I am hearing of issues of radiant heat and well insulated homes. I have read online:

“When the heating load is very small, the radiant slab has to be maintained at no more than a few degrees above room temperature to prevent overheating, and this means that the slab isn’t likely to be warm to the touch. A slab maintained at 74°F (23°C) will be cooler than an occupant’s skin, so bare feet will conduct heat into the slab.”

If this is true I’m not sure if radiant floor heat would be worth the extra cost over forced air. I am looking to create a comfortable environment especially not cold floors. Have people experienced this is in the real world?

BTW I am looking at a hydronic system, I know that an electric system will probably be too expensive to run. My real question is overheating a real problem in an energy efficient house? One where you can get the floors hot enough to feel warm as the house would be too hot and overheat.

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Pawan,
    Q. "Is whole house radiant floor heating effective in the real world?"

    A. Yes, it is effective.

    Q. "I would like to use radiant floor heating throughout, mainly for comfort but secondarily for hopefully lower energy bills."

    A. Radiant floor heating will not lower your energy bills.

    Q. "I'm not sure if radiant floor heat would be worth the extra cost over forced air."

    A. You are right to be skeptical. If you want to improve your comfort, the best way to spend your money is to upgrade your thermal envelope (by reducing air leakage, installing more insulation, and buying better windows). Once you have improved your thermal envelope, it hardly matters how you heat your house.

    Q. "Is overheating a real problem in an energy efficient house?"

    A. Summertime overheating can be a problem, especially if you install too many south-facing windows or west-facing windows. You need to design your house carefully to address this issue.

  2. Bob Irving | | #2

    We've built (non-passive-solar) homes heated by radiant floors; they work very well. Since then we've gone to superinsulated homes, which, as Martin says, are very easy to heat -and cool - with substantially lower cost heating systems. The last radiant heated home I built like this was 3500 Sf and cost about $2,000/yr to heat; With our present methods it would be 1/3 to 1/2 of that annual cost. The key is to upgrade your envelope.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Heat your master bath floor with radiant. Still a nice feature to wake up to.

  4. Expert Member
    Dana Dorsett | | #4

    As a general rule in new construction, for the cost of any radiant system more elaborate than PEX in a concrete slab, you buy more comfort (and energy savings) by directing that money on lowering the heat load.

    Even though it's possible in a moderate-R house at a lower room temp with radiant floors than with other heat emitters, what studies have been done on this demonstrate that the vast majority of the occupants of said houses run them at the same room temp they would if the were using crummy fin-tube baseboard, eliminating most of the energy savings. What savings can be gleaned at the same temperatures is in combustion and system efficiencies of the boilers & distribution plumbing over a higher-temp hydronic system. Assuming best-case design practices for both, using modulating boilers under outdoor reset control we're talking at best a 5% fuel & power use reduction by going solely with radiant.

    Ducted hot air has an energy use penalty over hydronic heat delivery for two primary reasons:

    1: The thermal mass of air vs. water, and duct-impedances vs. plumbing resistances means delivering the same amount of heat with air takes several times the power use in air handlers than would be used by pumps in hydronic systems.

    2: Even in the best well sealed Manual-D duct designs, the air handler pressurizes some rooms, depressurizes others, which drives outdoor air infiltration through the leaks in the building envelope, actually increasing the heat load whenever the air handler is operating. In "typical" new home duct designs & infiltration leakage it's on the order of a 10% hit in additional fuel use with the standard 2-3x oversized fossil burner with a single-speed air handler. With a modulating right-sized system these issues are reduced by a lot, but never completely eliminated. Air-sealing the house to under 1000cfm/50 can make a real difference on the size of the problem too.

    Hydronic heat deliver using low-temp panel radiators a fraction of the cost of most radiant floor systems, is QUITE comfortable, more responsive than most radiant floors to changes in load, and has none of the ducted air delivery issues.

    Ductless heat pumps are a small step down in comfort from there, provide very stable room temps, are POSSIBLE to right-size for a high-R house, and are quieter than all but the very best variable-speed ducted air solutions.

  5. rtp_130 | | #5

    I am curious as to some other possible benefits of hydronic heat that don’t seem to get mentioned here. The 2 main benefits that I enjoy are:
    1. Hydronic heat is very quiet compared to forced air. I live is the prairies of Canada and forced air system, even the expensive variable speed systems, run a lot in our winters and make a lot of noise.
    2. Forced air systems continuously blow dust around our homes, for me this is a huge detriment to forced air. Hydronic systems don’t have this issue.

    I am not familiar with heat pumps as I haven’t lived with one but from what I can tell from reading they wouldn’t work too well in our winters up here. We can easily get days on end where the day time high might hit -25 Celsius. So I am not sure if there is heat pump technology that would work for us.

    I am curious about this topic as we are in the process of building a new home. We want to focus on air tightness, reduced or no thermal bridging and as much insulation as we can afford. In the end due to builder experience, or lack of, we will probably go with a 2x6 traditional framed house and try to get it as tight as we can get the builder to do. Will then wrap with at least 2” of foam to help with thermal bridging. So this house is definitely not super insulated but it would be well above our code and we are hoping it would be comfortable. My big concern is what to do for a heat source. As you can tell from my earlier comments I am not a fan of forced air. Given our extreme winters up here would you guys say that radiant in floor would be wasteful in this house?

    1. DCContrarian | | #6

      The flip side is that forced air continually filters the air in your house, which radiant heat does not.

  6. Paul Wiedefeld | | #7

    Radiant floors won’t save any energy and will be significantly more expensive to install than forced air (which is split with air conditioning). If it’s gas fueled, it’s not green either. Hydronics have great economies of scale, but struggle with low load houses. A lot of expensive equipment/materials and a lot of specialized labor for not enough heating.

    1. rtp_130 | | #8

      How do you fuel your forced air, mine is natural gas. So I don’t understand?

      1. Paul Wiedefeld | | #9

        Central heat pump (not ductless), saves me 50% vs natural gas. Much more comfortable. Depends on your utility though!

  7. Jon R | | #10

    Radiant floors can save some energy over ducted air. Besides Dana's points, radiant floors almost always reduce the temperature delta needed, which makes heat pumps and condensing gas systems more efficient.

    Hydronics allow buffer tanks, making low loads a non-problem.

    Cost and complexity are legitimate downsides.

    1. DCContrarian | | #11

      I've seen t hat claim made but never documented. Certainly when you do a Manual J you don't make an adjustment for it.

      1. Jon R | | #12

        Manual J is about load, not efficiency.

        1. DCContrarian | | #13

          Manual J is used to size your equipment. If a heating method were more efficient you would downsize the equipment for that method. You don't.

          1. James Howison | | #17

            Is there any way to get a predicted CoP using a specific Manual J throughout the year? ie at different levels within the modulating range the system will operate at different efficiencies (aka CoP). So we should be able to take the loads, then calculate the % of time at each CoP, then do a weighted calculation.

            Currently you can have two systems both with the same max output (and thus indistinguishable for Manual J, no?) but one has a much lower modulation and thus lower minimum output ... surely that unit is more efficient through the year?

            I think the buffer tank is a version of that argument, no?

    2. Paul Wiedefeld | | #14

      I don't think the lower temp helps efficiency vs. forced air. A "high" efficiency furnace will definitely be condensing with room temperature return temps. Ditto with heat pumps.

      Radiant floors will usually help condensing boilers run more efficiently.

    3. DCContrarian | | #16

      The benefits you've laid out are of low-temperature hydronics in general, not radiant floors specifically.

  8. James Morgan | | #15

    Reply #6 already mentioned the air filtration advantage of ducted air systems, but even more significant is that climate change is likely to send the air conditioning boundary much further north, well into New England and beyond, and radiant cooling is not an option in any but the driest climates. Many homeowners in the north east already treat a/c as a necessity, either as an add on to a fossil fuel furnace or as an integrated high performance heat pump system.

  9. Robert Norris | | #18

    Homeowner two-cents: We’ve lived in North Pole, Alaska for the past 16 years, 15 years of which are in two different radiant floor heated homes. I understand that mini-splits are the state-of-the-art for best energy/comfort but as we design our next new home for retirement in the PNW it will be radiant floor heated, here’s why—
    1. Comfort: floor is warm. We have our condensing boiler set to 135-degree water with a delta T (20-degrees) single circulation pump. Our room temp is kept at 72-degrees
    2. Quiet
    3. No fans blowing air causing that “cold air effect”
    4. Minimal maintenance (no filters, outdoor components, etc. to maintain)
    5. No invasive room baseboard heaters, wall units, registers, etc, etc
    6. Many options to provide heated fluid: electric/propane/gas hot water heater tanks, on-demand water heaters, dedicated boilers
    7. Heat source can also be used for domestic HW

    Overall, I’ve been open-minded to the discussions here on GBA about best options for heat and I’m solidly set on radiant for the above reasons. You can’t beat it for comfort and convenience. If you’re worried about potential freeze-ups use 100% glycol and accept the slightly reduced efficiency.

    1. Paul Wiedefeld | | #20

      "6. Many options to provide heated fluid: electric/propane/gas hot water heater tanks, on-demand water heaters, dedicated boilers"

      This is the big one! Hydronics provide a lot of flexibility, which is awesome. Without commonplace air-to-water heat pumps though, they are not green yet. A boiler installed today pollutes for decades.

    2. James Morgan | | #21

      Hey Robert, don't be surprised if when you move into your new home 2,000+ miles south your concrete floors are no longer warm. If that is your home is insulated worth a darn. The warm floor effect diminishes substantially as your heating load decreases. You're going need your slippers more than ever. Just saying.

  10. Michael Madore | | #19

    We have a super insulated passive solar home that uses hydronic heating, the floor is cool to the touch and we leave it a comfortable temperature of 18.5 celcius. We've experimented with higher temperatures and found if we creeped up to 20 degrees, it was generally too hot. We use indoor footwear more because the concrete and tiled floors are bit hard and that mitigates having cold feet.

  11. Jon R | | #22

    Note that you can restrict the area where the radiant floor is heated. This results in it being warmer in those areas, restoring foot comfort at low loads. The higher temperatures will typically reduce efficiency.

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