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Community and Q&A

Radiant floor heating using heat pump?

JohnBoy28 | Posted in Mechanicals on

We live in a Zone 7 area and are building a one story 2000 sq ft on a slab and have decided we would like to heat the house with in-floor radiant. We have been told by HVAC specialists that the best solution would be geothermal, followed by hot water heat pump, electric hot water, then forced air heat pump and electric radiant. We don’t have natural gas so that is not an option. Geothermal is out as it is nearly three times the price of the other solutions and this is a retirement home.

The second solution we had not heard of. It was described as a heat pump with a hot water tank or boiler that heats or cools the water similar to how a forced air heat pump works but heating and cooling water instead of air.

I am waiting on pricing for this and the electric hot water solution. I found many old discussions on hot water heat pumps but they were small units to replace hot water tanks.

Is the order of solutions logical and what can you tell me about the efficiency and comfort of the hot water heat pump solution?

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  1. GBA Editor
    Martin Holladay | | #1

    First, let's clarify our terminology.

    You are contrasting "geothermal" with a "hot water heat pump." But a "geothermal" system is a type of heat pump.

    What some people call a "geothermal" system is more properly called a ground-source heat pump. A ground-source heat pump can be connected to a hydronic distribution system (one using hot water -- for example, in-floor hydronic -- sometimes called "radiant" -- heat) or a forced-air distribution system (using ducts).

    When you say "hot water heat pump," you are probably talking about an air-source heat pump like the Daikin Altherma. This is an air-to-water heat pump.

    An "electric hot water" system is more accurately called an "electric resistance water heater" or an "electric resistance boiler." (After all, all three types of systems we have discussed so far are electric.) It makes no sense, in my mind, to use an electric resistance water heater or electric resistance boiler to heat water for a hydronic distribution system. If you want electric-resistance space heat, use electric-resistance baseboard or in-floor electric resistance heaters.

    What you call a "forced air heat pump" is more accurately called an air-to-air heat pump. Some of these systems use ductwork; others (ductless minisplits) don't require ducts.

    You refer to "electric radiant," but as I said earlier, many of the systems we have discussed use electricity as a fuel. I think you are talking about "in-floor electric resistance heat."

    OK. We've clarified our terms. Next up: choosing between these systems.

  2. GBA Editor
    Martin Holladay | | #2

    Considering the wide range of heating systems under discussion, I suppose the first question we need an answer to is this one: Why do you want in-floor hydronic ("radiant") heat distribution?

    Before answering the question, you may want to read this article: All About Radiant Floors.

  3. JohnBoy28 | | #3

    Thanks very much for the detailed explanation for the various terms. This is the first time I have seen such clear descriptions.

    We are totally confused as to what type of system to install. We are on a restricted budget so solutions like ground source are out and as mentioned we do not have access to natural gas. We are looking for a balance between initial system cost and efficiency. Heating is not critical now as we spend our winters in Florida but as Canadians we will eventually find health care costs prohibitive and need to stay home in winter.

    Many discussions suggest building on a slab in cold climates will result in floors feeling cold and several posts also suggest that next to ground source, hydronic air to water heat pump is the most efficient. That is why we are looking at radiant.

    We get a few -35F days each winter so it can be pretty cold thus we are considering installing 4" of rigid foam under the slab vs 2" (code) to reduce heat loss.

    We are open to any solution. What would you suggest? We do need a little air conditioning but a duel zone mini-split could handle that.

  4. Expert Member
    Dana Dorsett | | #4

    The Daikin Altherma would run out of capacity in colder parts of Zone 7, and it's a bit expensive.

    An electric boiler is very cheap to install, more expensive to operate, and you may need one to cover the shortfall on the Altherma when it's -20F outside.

  5. JohnBoy28 | | #5

    Dana, For sure, I would need an electric boiler or tank for cold weather.

  6. Expert Member
    Dana Dorsett | | #6

    If you're not there in winter and experience -35F temps, you might consider a Fujitsu xxRLS3H air-to-air solution, which has a specified output at -15F, but still keeps running at lower temps (unlike the Mitsubishi competition, that shuts down at some temp below -18F) . You can use an electric boiler slaved to a floor thermostat to keep the slab from feeling cold (but only when you're there), and use the mini-split to maintain the room temperature (all the time). You can buy a LOT of RLS3H for the price of an Altherma- they're nice an quiet too.

    The Altherma has no specified output below -20C/-4F.

    With an electric boiler for the slab, setting the floor temp to about 0.5-1C above the room temp guarantees barefoot comfort. To make sure that the mini-splits are pulling the bulk of the load you may need to buy a hard-wired wall thermostat, since the setpoints will have an offset when it's just sensing the temp of the incoming air at the head.

  7. STEPHEN SHEEHY | | #7

    I'd forget the radiant floor. With a tight, well insulated house, you'll find the heating system often not running enough to keep the floor warm.

    I'm going to disagree with Dana. If the floor is 25 degrees F below body temperature, you may not get warm feet. A basic question is this: In winter, do you usually walk around in bare feet?

  8. JohnBoy28 | | #8

    That sounds like a reasonable solution but I am concerned about a mini-split running unattended for 6 months. My main concern would be if it ended up under a 5 foot snow drift. I realize the radiant system would kick in if the mini-split failed or could't produce heat but what about potential damage to the system.

    Your suggestion would defiantly meet my initial cost/efficiency criteria. I usually set back my thermostats to 50F while away and I will probably invest in an electronic one that will email me if the temperature drops. I also have someone checking the house weekly. That said, do you still think the mini-split would be a good choice?

    Are you aware if the PLS3H works with a dual zone duct system so we can mount the inside units in closets? . Wall mounted stand-a-lones will not work with our decor.

  9. JohnBoy28 | | #9

    No, we will not be barefoot and are fine if the floors are not super warm. We just don't want them to feel cold. Our flooring will be ceramic and engineered wood, no carpet. If we don't install radiant hot water we will need another back up system for the mini-split.

  10. JohnBoy28 | | #10

    Martin, Dana, Stephen:
    Maybe, as I mentioned, we should install 4" of rigid foam under the slab and reduce the worry of cold floors and then go with a full ducted system using an air to air heat pump. I assume that would be more efficient to run than an electric resistance boiler and mini splits. If I went that route I would probably install in-floor electric resistance heaters in the bathrooms.

    I have received some price estimates and it looks like the air to air would be the lowest cost system with the mini-split/ electric boiler $5,000 more and the mini-split /air to water heat pump boiler $15,000 more then the air to air.

    What are your thoughts?

  11. charlie_sullivan | | #11

    If you are not there most of the winter, the Fujitsu air-to-air solution Dana suggests would be great:
    1) The floor temperature comfort issue is not important when you are not there, so you can leave the floor heat off during that time.
    2) The heat pump gets even more efficient if you set the indoor set-point lower, e.g. to 10 or 15 C.

    Although an electric "boiler" for the slab is much less expensive than an air-to-water heat pump, I think that directly putting the electric heat in the slab is even cheaper. But I'm not sure of that. And putting water in the slab has the advantage that a future resident could install a different heat source for the water, whether that's a CO2 air-source heat pump, a wood-chip sterling engine CHP system or a newly-invented technology.

    But I think you could also skip the floor heat altogether. If you have the slab and the building well insulated, the slab will reach equilibrium with the room temperature. Whether the slab is unheated and at 19 or 20 C, or heated to 21 C, or even 23 C, makes surprisingly little difference in how it feels to your bare feet. If your feet are at 37 C, those different floors are 14 to 18 C colder than your feet. In all cases, heat is conducted away from your feet into the floor, and how cold it feels has more to do with the thermal conductivity of the floor surface material than a few degrees of temperature difference. If you insulate well and have a flooring material with moderate to low thermal conductivity, such as wood or carpet, you won't have a problem with the floor being too cold.

  12. GBA Editor
    Martin Holladay | | #12

    We're reaching a consensus. Insulate under the slab and skip the floor heat.

    If you go with an air-to-air heat pump connected to ducts, make sure that the system is rated for operation below zero. Equipment from either Mitsubishi or Fujitsu would work; you may even want to consider using Mitsubishi's new MVZ air handler paired with a Mitsubishi HyperHeat outdoor unit (although this approach isn't the most efficient). For more information on these options, see How To Buy a Ductless [or Ducted] Minisplit.

    If you're worried about keeping the house warm when the temperature drops to -30°F, a couple of electric resistance space heaters are all you need.

  13. JohnBoy28 | | #13

    Thanks very much. I think we do have consensus.

    Do the Mitsubishi or Fujitsu have the option of a electric resistance heater attachment to produce heat in very cold weather? My daughter in Florida has this on her heat pump and used it once when the condenser on her unit failed.

  14. GBA Editor
    Martin Holladay | | #14

    Q. "Do the Mitsubishi or Fujitsu have the option of a electric resistance heater attachment to produce heat in very cold weather?"

    A. No. But you can buy an electric-resistance space heater at any hardware store -- or at Wal-Mart, for that matter.

  15. Dana1 | | #15

    The problem with a space heater as backup for when you're in Florida is that they don't come with room-thermostat control.

    Installing an option to run the electric boiler on a room thermostat could work as the backup. If you turn the setpoint on the thermostat 305C below where you set the mini-split (eg. set the mini-split to 13-15C, and the hydronic system to 10C) the hydronic system only comes on when the mini-split is unable to keep up with the load.

  16. user-2890856 | | #16

    Before making any decisions on systems I believe one aspect of your situation is being overlooked . John , do you and your spouse intend to as we say , " age in place " in this home ? Nobody has even addressed this concern . As folks get older , the ability to tolerate lower temps or temperatures that one readily tolerated before becomes a real issue . Many older folks start to see the need for heat at higher outdoor temps as they advance .

  17. JohnBoy28 | | #17

    Space heaters will not work if we get several days at -30F and try to heat 2000 sq ft. If we can't find a heat pump system or in duct electric heater that can provide electric resistance heat to the duct system we will have to install a secondary system like radiant electric or baseboards. I would prefer not to have to do that.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    Another poster, (I think it was Hobbit?) faced the same problem. He installed easily demountable wall heaters that he removed in the summer. I'm sorry I can't find the thread - it was about two months ago.

  19. GBA Editor
    Martin Holladay | | #19

    I can understand your reluctance to depend on a minsplit system in Climate Zone 7. However, many homeowners are following the path suggested here.

    For the approach to work, you need the pay attention to the thermal envelope of your new home, making sure that insulation levels are higher than minimum code requirements; that air leakage rates are as low as possible; and that high-performance windows are specified.

    Installing enough linear feet of electric-resistance baseboard heat to help your home ride through the occasional cold snap is not particularly difficult or expensive.

    That said, if the approach makes you nervous, you can install a propane-fired furnace or an oil-fired furnace if you want.

  20. JohnBoy28 | | #20

    I am definitely leaning towards your suggestion. We will be paying attention to the thermal envelope with 6" walls and possibly spray foam insulation, extra insulation in the attic and quality triple pane lowE argon windows, plus 4" of rigid foam under the slab.

    I will get pricing next week on radiant in-floor electric resistance heat. If not too expensive I could use this as a backup and set it lower then the heat pump. When away for the winter I could set the heat pump at 12C and the radiant at 8C. On very cold days when the heat pump can't provide enough heat the radiant would take over. When we stay home for the winters we could set it a few degrees below the heat pump setting.

  21. GBA Editor
    Martin Holladay | | #21

    That approach will work, although electric-resistance baseboard units are likely to be less expensive than in-floor electric heat.

  22. JohnBoy28 | | #22

    A local HVAC contractor came up with the following recommendation for a ducted air to air heat pump system.:
    LG LMU480HV 4Ton outside
    LG LMVN360HV inside
    Vannee 100h air exchanger
    10 kw heater

    I would add electric resistance radiant in the bathrooms only. Does this sound like a sensible solution?

  23. Dana1 | | #23

    What is your heat load at the 99% outside design temperature? Without knowing the load and the design temperature, it's hard to say for sure what does or does not make sense. Picking a heating solution ahead of that information is dubious approach. That said...

    The 4-ton ducted LG is substantially less efficient (HSPF=9.7) compared to a cold-climate ductless Fujitsu (HSPF=13 to 14+). The ductess Fujitsu would deliver more than 30% more heat per kwh than the LG under most conditions. They don't even specify an efficiency or capacity for the ducted LG at +17F/-8C, let alone at -25C (see page 28):


    It would make a nice air conditioner, but you actually CARE about it's heating capacity at temperatures of -25C or lower.

    A cold climate Fujistu ductless has a specified capacity at -15F/-27C:

    The extended temp heating capacity tables start on page 15(pdf pagination) :

    (The last document is for the prior generation RLS2H series. The RLS3H series units have comparable or better capacity, and significantly better efficiency.)

    Even if they eventually test it's low outdoor temp capacity, unless the LG has automatic pan heaters to deal with defrost ice build-up in cold climates you probably wouldn't even be running it on days with average daily temps below -5C to avoid damaging it.

    A Quebecer who sometimes posts here under the handle "Jin Kazama", heats his place with four Fujitsu (_)RLS2H series mini-splits. You might be able to search some of his relevant posts about heating with ductless in his zone-7 type climate.

  24. JohnBoy28 | | #24

    D Dorsett:
    Thanks very much for the feedback. The HVAC company did a heat load calculation but didn't share the results with me.

    I looked at the numbers for the Fujitsu and they are for a stand-a-lone unit while the LG is for a full duct system. I know the single units are more efficient but large wall mount units will not meet are requirements. (wall vents only)
    I will talk with the Fujitsu dealer and look at the quebec posts.

  25. Dana1 | | #25

    How do they do a heat load calculation without knowing all of the specifics of the wall construction & insulation and the window specs?

    "We will be paying attention to the thermal envelope with 6" walls and possibly spray foam insulation, extra insulation in the attic and quality triple pane lowE argon windows, plus 4" of rigid foam under the slab."

    If you are building to better-than-code-min your loads will be substantially lower too. A 4 ton system for a 2000' house is a LOT of compressor- serious overkill for air conditioning. The only way you'd need that much is if the thing falls flat on heating capacity at lower temperatures (which it apparently does, if doesn't even have a specified capacity at a temp as moderate as -8C)

    Fujitsu's- _ _ RLFCD mini-duct units aren't quite as efficient as the wall coil types, but they are are fully specified down to -21C, and more than 10% more efficient at moderate temps than the 4 ton LG:

    It might take more than one, but people usually prefer to be able to have different zones, which cuts into efficiency if it's done on a single compressor single air handler system.

    An air tight 2000 square foot 2x6 framed house with open cell foam cavity fill and an inch of rigid rock wool insulation on the exterior, with triple pane windows and R50 in the attic will have a heat load of something on the order of 25-30,000 BTU/hr @ -25C give or take, and could have heat load as low as 20K with minimal window area and attention to air sealing. A single pair of the 15RLS3H would put out 30KBTU/hr @ -25C, but it may take a pair of 18RLFCDs to get there with mini-ducts. (I don't have access to the extended temp tables on the mini-duct versions.) That's still 2.5-3 tons of compressor- a lot, but substantially less than 4. Buy unlike the 4 ton LG, there are capacity specs at or near your 99% outside design temps to work from.

    The same house with 2x6 / R20 and no exterior insulation, with code-min double panes and R30-R38 in attic might be closer to 40,000 BTU/hr, which may be why they proposed 4 ton with unknown low-temp capacity on you.

  26. JohnBoy28 | | #26

    D Dorsett:
    Thanks again for all the time you have spent with the above advise. It helps me to better understand the numbers and to make an informed decision.

    Our code is R20 in walls, R 52 in ceiling and 2" of rigid foam under the slab. I gave the company my window sizes and mentioned triple pane low E.

  27. Dana1 | | #27

    An R20 stud wall 16" o.c. has a whole-wall performance of about R14 after thermal bridging, a U-factor of about 0.071.

    Adding R4 (about an inch) rock wool to the exterior increased whole-wall performance to about R18, a U-factor of 0.056, reducing the heat loss of the wall surfaces by about 21%.

    Adding R8 / 2" of exterior rock wool increases the whole-wall R to about R22, a U-factor of 0.45, a reduction of wall losses of 37% below code min.

    (A 2x6 R20 wall doesn't meet IRC 2012 code for US climate zone 6, let alone zone 7, which would require R5 continuous insulation, or about 1.25" of rock wool over the sheathing: )

    When reviewing heat load calculations watch out for large thumbs on the scale. Air infiltration numbers almost always overstate reality, often by over 300%. If you're building a house sufficiently tight to test at 3 ACH/50 or less in a blower door test (IRC2012 code max) almost all generic tool defaults exaggerate by huge margins. But it's also common for HVAC contractors to assume code-minimums (or worse), even when you've given them the specs on the windows and wall stackup.

    On any proposed heat pump solution, insist on seeing the extended temperature capacity tables, independent of it's nominal cooling performance BTU numbers. It's an eye-opener. Many heat pumps are useless below -15C, and some don't have a specified capacity below -10C, making them wholly inappropriate for a zone 7 type climate.

  28. user-1072251 | | #28

    John: The slab must not touch cold concrete or ambient air; otherwise it will draw heat out of the slab, so it's important that you not only isolate under the slab, but you also need to insulate the slab edges. With no edge insulation, you will lose heat to the cold foundation or outside air (depending on the exact location of your slab in relation to the concrete wall); this is called a "thermal bridge".

  29. JohnBoy28 | | #29

    Thanks Bob

    The plan is to have a 2" rigid foam thermal break between the edge of the slab and the foundation as well as down the inside/side of the foundation. We will also have 4" of rigid foam under the slab.

  30. GaryJanL | | #30

    Very helpful post. We have a nearly identical home (similar size on a slab) and switched from a propane boiler to electric after installing solar panels. We go -20 F in the winter at our home and had considered a heat pump or possibly adding more solar capacity. We have radiant in floor heating and use a propane furnace to help move the air and keep the house comfortable without using all of our electrical credits. After reading this thread, we will probably elect to add additional solar capacity instead of going to a heat pump. Thanks for a great discussion of the various options.

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