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Air source heat pumps and radiant floors

user-7552164 | Posted in Energy Efficiency and Durability on

We are designing a house with radiant heating in a town that is considering banning fossil fuels in new construction. Our hvac engineer has told us that he has not had good experiences using heat pumps in conjunction with radiant. Is there a good air source heat pump to use in this situation?

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  1. Expert Member
    Dana Dorsett | | #1

    LG 's smaller single-phase full VRF Multi V s systems with a hydro kit can work pretty well with low-temp hydronic heating, and would also be able to air condition using their typical mini-split and ducted air handlers (or chilled water radiant.)

    The compressor sizes in that series run from 2- 5 tons, but I believe even the smallest hydro kit would need a 4 or 5 tonner, and probably a buffer tank.

    It's a real PITA to surf for information on it on LG's websites, but the local/regional LG distributor might be able to dig it all out for you.

    See also:

    Appropriately sized reversible chillers such as Chiltrix or Arctic can work pretty efficiently with low temp hydronic systems too:

    With the reversible chillers the refrigerant loop is all in the outdoor units, and the connection is hydronic, which will require freeze protection (or antifreeze) in cooler climates.

  2. rhl_ | | #2

    Some people here are reasonably well connected to Mitsubishi. Is it possible to do this with Mitsubishi hardware? If not someone should point this example out to them as a major (and growing) market hole.

    Natural gas was banned in lower westchester. NYS is trending away from fossil fuels.

    1. krom | | #12

      Was natural gas actually banned? or did the gas company stop doing new hookups because NY'S screwed up government has mad it impossible for them to add the required capacity to support any more customers?

  3. Expert Member
    Akos | | #3

    You can look at the Sanden CO2 water heater for space heat and DHW. A single unit could do a small high efficiency house, a pair something larger. An open hydronic system with a large buffer tank is fairly straight forward, much less complicate than any reversible chiller setup.

    You can install a couple of high power elements in the tank as backup and low temp boost. Their tanks are very expensive but apparently you can use other ones provided it has enough stratification (some solar ones work).

  4. user-7552164 | | #4

    Thank you all, this is very helpful.

  5. Jon_R | | #5

    The Sanden isn't really designed for efficient space heating. They say "It is permitted to use the SANCO2 system to provide some limited capacity heating" (pretty vague). And it lists a minimum temp of 130F (an efficiency issue). It has a -15F outdoor temp limit (about the same as Chiltrix).

    With only two in stock, I wonder how serious Arctic is about the cold climate hydronic heat pump market.

    1. Expert Member
      Dana Dorsett | | #6

      I agree completely that getting a CO2 compressor water heater to function efficiently in a space heating application presents special problems that even the engineers are still trying to get right, whereas an R410A refrigerant reversible chiller is a known and knowable commodity that a reasonable hydronics designer can work with.

      1. Expert Member
        Akos | | #7

        Sanden says in their PDF ( ) that the unit can do combi duty provided there is a min 25gal of freshwater consumption per day and 8kbtu max space heat. That is not a lot, doable for something small though.

        1. Jon_R | | #8

          Just don't skip showers or go on vacation

        2. pnwbuilder | | #9

          I am curious about what drives minimum 25 gal/day requirement?

        3. Expert Member
          Dana Dorsett | | #10

          In the experimental systems in eastern WA from which those numbers were derived (a warm edge of zone 5B climate) the seasonal efficiency of those systems was about 2 (~HSPF 6.28), below the minimum required for space heating heat pumps in the US. And that was with engineering academics designing and monitoring & maintaining the systems!

          There is a Sanden based combi available commercially in France- a milder climate, where resistance electricity is common, and ANY COP>1 is an improvement over the status quo.

          CO2 heat pumps can only operate efficiently at fairly high delta-Ts- there is no phase change to take advantage of- it's only high pressure gas/low-pressure gas. Raising water from 40-50F to 120-130F is pretty efficient, but at the in to out delta-Ts in a hydronic heating loop are a few 10s of degrees F at most. So while the heating system can sip heat off the high-temp end of the tank, it's only efficient if the heat pump is creating that higher-temp water heating domestic hot water from a much lower temp, providing a much bigger temperature lift.

          1. Expert Member
            Akos | | #11

            I'm a bit surprised at the low efficiency. I know CO2 systems need low return temperature, but a well designed heated slab floor should get it close to 80F to 90F at which point the COP is not much less than at 60F (Figure 7) (

            Maybe best to tack a pre-heater to the ERV fed by the return form the floor heat to get the temps down even further.

          2. Jon_R | | #13

            This supports EF < 2 in actual use:


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