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Radiant system suggestions for a small home?

LouieR | Posted in Energy Efficiency and Durability on

Howdy all,
I’m building a 900 sf home in Maine and have installed 1/2″ pe-rt lines in the insulated slab (on grade) which will be my finished floor. Heating will be split between the slab, and a modest wood stove. I’m planning to install PV panels and am curious if anyone has thoughts on a good electric system to heat the floors. I had originally planned on a propane combination unit seeing as size is a major consideration, but would like to use electric instead.
Thanks for any insight!

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Replies

  1. Jon R | | #1

    Maybe a Chiltrix heat pump.

  2. GBA Editor
    Martin Holladay | | #2

    Louie,
    It sounds like you are looking for an electric boiler or electric water heater. In general, my response to someone who wants the type of system you are installing -- a hydronic system with an electric resistance boiler and hydronic distribution -- is that you are getting the worst of both worlds. You are getting the high fuel prices that come from using electricity as your fuel without the simplicity that comes from electric resistance heaters. You're getting all of the complications of a hydronic system -- pumps, expansion tanks, etc. -- without the low fuel bills.

    Jon R.'s suggestion -- using an air-to-water heat pump -- lowers your fuel bills but adds another layer of complexity (buying and installing equipment that local heating contractors will be unable to troubleshoot or repair).

  3. LouieR | | #3

    Should I be considering a different fuel source?

  4. GBA Editor
    Martin Holladay | | #4

    Louie,
    The vast majority of hydronic systems use natural gas, propane, or fuel oil as the fuel for the boiler that provides the hot water.

    If you want to use electric-resistance heat, the usual approaches would include something much simpler than a hydronic system -- perhaps electric resistance baseboard units or electric resistance panel heaters, without any circulating water in tubing.

    If you want to use an air-source heat pump -- thereby using about 30% to 50% of the electricity required for electric resistance heating -- the simplest way to heat your house is with a ductless minisplit, not with a Chiltrix air-to-water heat pump connected to a hydronic distribution system.

    1. Expert Member
      Dana Dorsett | | #9

      In most of Maine burning propane is comparable to resistance electricity, or not a lot cheaper. The smallest oil burners out there are ludicrously oversized for the space heating loads, and maintenance is high. An electric boiler is comparatively dirt-cheap up front.

      Spend the "real" HVAC money on a 3/4 ton cold-climate mini-split, which in my neighborhood runs ~$3k fully installed, give or take a few hundred. The electric boiler would run another ~$1K, and can be DIY installed if you're handy with both electrical and plumbing. Size the boiler for the design heat load, but control it with a floor thermostat set to a degree or two above the desired room temp, then use the mini-split to conrol the room temp.

      The Mitsubishi FH09NA is good for about 10,000 BTU/hr @-5F but modulates down to 1600 BTU/hr @ +47F, and will have the most stabler room temperatures during the shoulder seasons.

      The Fujitsu -9RLS3H delivers a hefty 14,000 BTU/hr @ =5F (which is probably more than your design load at that temp), but only modulates down to 3100 BTU/hr @ +47F, which means if you're running the radiant floor when it's that warm out it'll be cycling on/off a lot.

      Either one would be delivering more than 3x the heat per kwh than the radiant floor when it's above freezing and about 2x as much per kwh when it's 0F outside.

  5. LouieR | | #5

    Thanks for your response!

  6. Trevor Lambert | | #6

    Here are a couple of advantages to hydronic vs pure electric.
    1. You are not locked in on fuel source, it can be changed if one becomes more practical at a later date.
    2. You can also cool with it (yes, this definitely increases the complexity a fair bit)
    3. With a house as small as yours, you can easily heat the floors using your domestic hot water tank. One pump and one relay is about all you'd need, as I'm sure one zone would suffice. (Expansion tank only if you're on municipal water). If you got a heat pump water heater, you'd get some free cooling in the summer. With that size of house, some super insulation and proper shading / window selection, that might be all the a/c you need.

    One advantage of radiant vs minisplit: silence. This might be a non-issue for most people, but my wife has become so enamoured with the lack of any forced air sounds in the house, I am going to have a hard time convincing her that we should add a minisplit for efficiency and a/c.

    1. GBA Editor
      Martin Holladay | | #7

      Trevor,
      You wrote, "If you got a heat pump water heater, you’d get some free cooling in the summer."

      Yes, but you can't use a heat pump water heater for space heating, unless (a) you put the water heater outdoors (which is only possible in a very warm climate), and (b) the heat-pump water heater is very large or your design heating load is very low.

      In most cases, the heat pump water heater is indoors (and that's what you are assuming, too, if you are talking about free cooling in summer). You can't steal heat from the indoor air in order to produce heat to keep your house warm.

      1. Trevor Lambert | | #8

        I should have said hybrid heat pump. Most of the ones on the market are hybrids (Steibel Eltron is the only one I'm aware of that is not). So in the heating season, you would use pure electric. In the summer, you would use hybrid mode, in which it would prioritize the heat pump and make up any shortfall with electric. For a larger house, having a second tank in series would make sense. But for 900 square feet and no basement, the space heating and DHW load would likely be small enough to get by with just one.

      2. Jon R | | #21

        > You can't steal heat from the indoor air in order to produce heat to keep your house warm.

        But you can steal heat from the indoor air to keep the floor warm. A HPWH is the only solution for someone who wants to heat with something else (eg, a mini-split) but wants warmer than air temp floors.

    2. Jon R | | #26

      Hydronic can also enable controllable thermal storage. Potentially useful if you have a significant time-of-use cost difference.

  7. Expert Member
    Akos | | #10

    A low temp ducted heatpump water heater should be able to keep your floor a couple of degrees above room temperature most of the year. It would not be enough to heat the place, so additional heat such as the mini split or wood stove would be needed. Should be enough to take the edge off cold floors.

    If you can go with an open system (https://www.radiantcompany.com/wp-content/uploads/RR-D-T-MUNI.jpg), you can get both hot water and your floor heat with just a couple of additional parts. If you turn off the floor heat in the mornings before the big water draws happen, should be able to handle both, the only question is long the heat pump heater will last.

  8. Scott Wilson | | #11

    How are you heating your DHW? If you plan on having a small wood stove in addition to your radiant floor heating why not get a small wood fired indoor boiler (put it inside a small shed near the house and run the piping underground) and heat both the floor and the DHW with one unit? You're already cutting wood for the wood stove so why not use the same fuel for everything?

  9. LouieR | | #12

    Still considering options for potable water, though I'm leaning towards on demand. I would like to have a relatively user friendly system with the intention that someone staying in the house wouldn't necessarily need to use the wood stove. This possibly rules out a heat pump water heater? May revert to a propane combi unit but would much prefer an electric solution considering we will be installing solar panels.

    1. Expert Member
      Dana Dorsett | | #13

      An electric on-demand sufficient to run a 2.5 gpm shower in Maine requires a dedicated 150A/250VAC breaker for just the water heater, and it's abusive to the power grid.

      A propane combi heater's MINIMUM fire output would typically be more than your 99% design heat load for a code-min 900' house, and the fuel cost in much of ME is more expensive than electricity. A radiant slab would avoid the worst of the short cycling, but it's not a very cost effective solution in the end. An electric boiler &/or a mini-split is cheaper heat and lower maintenance.

      There's nothing wrong with a plain old electric tank for hot water. Cheap & reliable, doesn't gulp a lot of juice all at one time.

    2. GBA Editor
      Martin Holladay | | #14

      Louie,
      If you take Dana's suggestion and install a ductless minisplit, then you already have a way to heat and cool your house. At that point, you can abandon the PEX tubing installed in your slab, and rejoice that you didn't spend any more money for your hydronic heating system, which is unnecessary.

      The ductless minisplit answers your requirement for "a relatively user-friendly system" for people who don't want to light the wood stove.

      Q. "Does this rule out a heat pump water heater?"

      A. A heat-pump water heater can provide domestic hot water, but not space heat (since it will grab its heat from your indoor air). For a small, simple, house, though, an ordinary electric-resistance water heater makes more sense.

  10. LouieR | | #15

    I should possibly add that my walls will be R-37 and roof R-60. Another piece of info is that I've sold my partner on concrete floors by agreeing to heat them...

    Would an open system running a 18kw on demand unit be a reasonable solution? Would an open system run off a tank style (looking at Rheem Marathon) be out of the question? I would love to use a tank which I would be comfortable doing maintenance on, though I believe it would struggle to keep up with cycles.

    Thanks for all the help!

    1. LouieR | | #16

      My pe-rt lines hold just under 4 gallons.

    2. Expert Member
      Dana Dorsett | | #18

      18kw x 3.412 BTU/watt-hour = 61,416 BTU/hr. That's probably on the order of 6x your actual heating load. It's a terrible solution for space heating.

      A 1.5 gpm low flow shower is 1500lbs/hr. 61,416 /1500lbs=41F temperature rise. So when the incoming water is under 40F you showering temp will be no more than 80F. Definitely not your hot water solution. The Marathon works.

      Why not calculate your actual heat load?

      A dumber than a box o' rocks 2.5 kw electric boiler (eg: http://www.electromn.com/pdf/BL308.pdf ) costs less than a grand, and delivers over 8,500 BTU/hr a 4kw electric boiler isn't much more money, delivers over 13,500 BTU/hr. You probably don't need more than 5kw (still under a grand for the really dumb ones) even with the windows cracked. Unlike an on demand water heater it's designed for the duty cycle & flow of a heating system. For a few hundred more you can even get electric boilers with outdoor reset control.

      A 3/4 ton mini-spit costs about three grand, but uses less than a third the power of the electric boiler (averaged over the season.) If you controlled the electric boiler with a floor thermostat to keep it nice & cushy and controlled the room temp with a mini-split you can have a bit of the best of both worlds. However, in a high-R house you won't necessarily feel that barefoot warnth very much until it's below 10F outside even if running just the boiler for heat.

  11. Expert Member
    Peter Engle | | #17

    How about installing a small electric resistance WH with a circulating pump, and letting it do as much as it will do. With a well insulated house, you don't really want a "warm" slab anyhow. Pump a few kW into it, and that'll keep it a degree or two above room temp, assuming you've got a few inches of insulation under it. Then you've met the partner's requirements to heat it. Nobody said it had to be heated well.........

    Follow Dana's advice for the "real" heat, and use a minisplit. Then you have A/C for the summer when needed.

  12. Stephen Sheehy | | #19

    I'm in Maine in my bare feet on my concrete floor right now. Insulate the slab with four inches or so of reclaimed foam and take the advice from Dana and Martin and use a single minisplit for heat and occasional cooling. For a low load house like yours, a heated floor will rarely be warm enough to keep your toes toasty while not overheating the space.

  13. Jon R | | #20

    A few degrees warmer supposedly makes very little difference in comfort. Far more important is the flooring material.

    http://www.healthyheating.com/Thermal_Comfort_Working_Copy/Definitions/floor_temps.htm

    1. Stephen Sheehy | | #22

      Jon - we have Ditra heat under our tiled bathroom floor. At 75°F, it feels noticeably warmer than when the heat isn't on, when the floor temp is usually about 68°. I'd suspect that in a well insulated, tight house, using a wood stove as the primary heat source, adding enough floor heat to warm the floor appreciably would tend to overheat the overall space.

      1. Jon R | | #24

        I agree, barefoot vs "normal footwear" makes a huge difference. Bathroom rugs and heating bathroom floors (preferably only in areas where feet will be) makes comfort sense.

        I also find that instant radiant heat (eg heat lamps) helps with bathroom comfort without the cost of always keeping it warmer.

      2. Trevor Lambert | | #25

        In a well insulated, well sealed house of this size, a wood stove alone will overheat the space.

  14. Expert Member
    Dana Dorsett | | #23

    >"... my walls will be R-37 ..."

    That means different things depending on context. R37 at center cavity isn't the same performance as R37 "whole wall".

    A low density R38 compressed into a 2x12 cavity performs at R37, but at typical framing fractions & sheathing/siding details has a "whole wall R" of about R30.

    A 2x6/R23 rock 24" o.c. + 2.5" of exterior continuous polyiso is about R37 at center cavity, but a "whole wall R" of about R33.

    A 2x6/R23 wall with 3.5" of exterior polyiso has a whole wall R of about R37.

    Any of them are fairly high performance, and window performance will likely dominate the heat load numbers assuming you get religious about air sealing.

    Calculate the U-factor of your walls using this as guidance, a datum necessary to calculate the heat load:

    https://www.greenbuildingadvisor.com/article/all-about-u-factor

    Then calculate the heat load using IBR methods to get a decent handle on just how much equipment it would take:

    https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1

    https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-2

    Use the window manufacturer's U-factor for the windows, and assume the ceiling/roof U-factor will be somewhere around U-0.020, give or take, possibly less if it's a trussed roof with the bottom chords buried in insulation.

    Most cast iron wood stoves could turn a place like that into a sauna pretty quickly, but high-mass masonry/stone "Russian stove" or "rocket stove" type woodburner can be pretty decent for heating a low-load home. With the high mass stoves it may only get fired once per day, the timing and amount determined empirically. The compromise would be a small soapstone woodstove fired intermittently, the thermal mass of the soapstone leveling out the peaks a bit. The house has some thermal mass, so it won't go from 65F to 85F instantly, but an oversized wood stove throttled back to 10% of it's max firing rate is more polluting and inefficient. Small but intermittent hot fires in a soapstone stove can run reasonably cleanly and efficiently without give you hot-flashes.

    If your partner insists on some heated floor, low-voltage electric radiant mat solutions in the few locations where it matters most could check that box. With only a smaller fraction of the floor heated warm enough for the warm toes feel won't necessarily overheat the place the way a full-on radiant slab is likely to.

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