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High Pressure Heat Pump (AWHP) Recommendations

4lane | Posted in General Questions on

Hi! I am replacing my gas boiler, in a cast iron radiator hydronic system. LG’s Therma V and Daikin’s Altherma claim to output hot water temps of up to 180 degrees, matching my current boiler. The thesis is, can I move off of fossil fuels in my cold climate (NY) 100yr old home by replacing the gas boiler with a high temperature AW heat pump and leverage the existing the aspects of the system such as cast iron radiators? There was a post on the very same topic three years ago, and 3 years later I also can’t find any installers or manufacturers of these systems. Are there any current High Temp AWHP options in the US?

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  1. paul_wiedefeld | | #1

    Yes, but it's difficult. The Caleffi Idronics series has lots of info on how to do this.

    Roughly, you need to find the heat loss of the house then divide by existing radiation, like this:

    You can find A2W units that go up to 120-130F from Nordic, Spacepak, etc. If that's the highest temp you need, you're done!

    If not, you have options which are not mutually exclusive:
    1. Reduce heat load to lower water temperature needed
    2. Add emitters to lower water temperature needed
    3. Use electric resistance to make up the shortfall for the fraction of the hours you need it
    4. Use a water-to-water heat pump to take your 130 water up to about 160.
    5. Keep the existing boiler for backup.
    6. If you have central air, use a ducted heat pump to supplement.

  2. 4lane | | #2

    I was looking at this article earlier and will give estimating my building's heat load a shot. And, my gas boiler will be a backup.

    So...LG/Daikin, etc's high temp AW heat pumps are not available in the US?

    1. paul_wiedefeld | | #3

      Not that I've seen. Very few Americans use boilers for heat. The Taco M-system comes closest to a drop-in replacement. But if you're keeping the gas boiler, it might not be that relevant. Finding out your heat loss and your EDR, and then Heat loss/EDR would help narrow down how much gas you could eliminate. There’s an easy way to get to about 130F water with air to water and up to 160f with some more complexity.

  3. Expert Member
    Dana Dorsett | | #4

    Where in NY? The Adirondacks are a heluva lot colder than the Hudson Valley, and two whole climate zones cooler than Long Island. A local 99% outside design temp of -15F is a lot tougher for a heat pump to handle than a design temp of +15F!

    LG's Multi-V series (full VRF heat pumps) has both low temp (up to 50C/122F) and high temp (up to 80C / 176F) Hydro Kit options, but to get any efficiency or capacity out of them will require being able to deliver design-day heat with <120F water, <110F even better. The biggest of the single-phase Multi-Vs maxes out at 5 tons, which is enough for most insulated homes with double panes at design temps down to -5F or so, but if you're running a high temp Hydro Kit you'll likely need something bigger, and those require 3 phase power. A short sheet spec on the Hydro Kit options lives here:

    More info on the single phase Multi-Vs can be dug out here:

    Just because you're running your current boiler at 180F doesn't need you NEED 180F output to make the system work with your radiation & climate. Most cast iron boilers get set to 160-180F primarily to avoid destructive condensation of acidic exhaust on the cast iron heat exchanger plates. I'd hazard that more than 3 out of 4 homes heated with 180F cast iron could be heated just fine (and more comfortably) with 140F or cooler water, especially if the house has been upgraded with air sealing & insulation and better windows at some point after the original system was installed.

    Many 1920s homes were fitted with heating systems capable of heating with the windows cracked 2" for ventilation, an artifact of public health policy recommendations for higher ventilation after the 1918 flu pandemic. If any of your radiators are painted with a semi-metallic silver or bronze paint, it's an indication that they were oversized and putting out too much heat for comfort when the windows are closed. Painting those rads with any non-metallic paint would give them ~15-20% more output (at any water temp), making it more likely to work at low temp.

    I've heard through the grapevine that the Massachusetts Clean Energy Center will be coming out with a new searchable list of heat pumps & installers, which would would be able to filter for hydronic output heat pumps. That won't do you any good in NY, but it might point you to options that are at least available in the US. I'm currently in the market (in MA) for retrofitting my own 1920s home, and it's been slow going so far.

    SFAIK the LG Therma V only comes as a monobloc, not split (an issue in places with very deep freezes), and isn't really available here. Daikin's Altherma was pulled from the US market after a very tepid product launch a decade or so back. (The Altherma comes both in fully-split and monobloc versions in Yurp. The LG Multi-V only comes a split system.) Chiltrix, Arctic, and SpacePak, and Enertech are all available at least SOMEWHERE in N.America, but only in monobloc configurations.

    The Mass CEC put together a series of seminars covering hydronic air source heat pumps a few years ago, now available on YouTube, well worth the time when considering this type of retrofit:

    Enertech also has some marketing and market-development video of interest online here:

    Слава Україні!

    1. 4lane | | #5

      You and I are on the same path. We should team up on a proposal to our respective states to explore AWHP in colder Northeast climates. I am in the Hudson Valley, further south than you, but considering the amount of older homes with steam/hot water boilers in this region, I can see how the states would be interested to have an option to convert these homes to AWHP over time. In NY, we have NYSERDA, whose mandate is just that.

      I am in the process of broader electrification of my home, using solar to get off fossil fuels (in my case natural gas) so my current drive is to replace my existing hot water heater and ancient gas boiler with a single AWHP plug and play solution. But I can't find any installer even willing to entertain anything that isn't already probably sitting in their warehouse. Which are Naviens, at the most progressive end of the spectrum.

      I calculated heat loss and EDF - first time doing this so might want to verify I have everything correct. How can I use this to assess whether existing AWHP options meet this need?

      Outside design temperature: 9
      Gas usage (Jan 19-Feb 16): 301
      Input BTU/h rating: 175000
      Output BTU/h rating: 140000
      Thermal Efficiency: 0.8
      Heat delivered (BTUs): 241
      Convert Therms to BTU: 24.1 M
      Base 65°F heating degree-day: 1158
      Base 60°F heating degree-day: 1023
      BTU per degree-hour at a balance point of 65°F: 866
      BTU per degree-hour at a balance point of 60°F: 980
      Heat load @ 65°F: 48508
      Heat load @ 60°F: 50000

      Radiator EDF (total): 737
      Radiator BTU (total): 125299

      Heat Loss/EDF: 65.81

      1. paul_wiedefeld | | #6

        Beautiful! That's a lot of radiation! Your typical heat emission chart of average water temperature vs. BTU/hr per sqft EDR shows that 130 degree average water temp provides 70 btu/hr/EDR sqft and 50 btu/hr/EDR at 120 average water temp. So a 48,000-60,000 btu air-to-water heat pump that produces 130 max is probably unable to 100% meet your needs, but might get you 90-99% of the way there, since heat load is not evenly distributed. The gas boiler can take it from there. Having that much radiation makes this all possible. All that said, finding an installer will be no small feat. On the positive side, since the heat loss is high enough to maximize the usage of a heat pump's output and your climate is cold, the installation cost will be spread out over a lot of MMBTUs.

        Source for those btu/EDR numbers:

        1. 4lane | | #7

          Trying to follow your train of thought here. My Heat Loss/EDF ratio 65.81, which you're equating to average water temperature vs. BTU/hr per sqft EDR. So, if 130 degree water is at 70, I'm at 68 during the coldest period of 2022, then you're judging that a awhp producing 130 degree water would suffice for 90-95% of the winter heating needs. And you're sizing a 48k-60k btu unit based on the heat load @ 65°F value of 48508. Am I following you?

          1. paul_wiedefeld | | #8

            That’s correct. 70 is greater than 68 of course, but this is average water temperature not supply water temperature. So 130 leaving the heat pump and 120 returning to the heat pump would give you an average of 125.
            The sizing is also correct but with a caveat - I mean 48,000 btu heat pump capacity at 9 degrees outside, not how manufacturers rate their units in the marketing literature. All this is possible because of how much EDR you have.

        2. Expert Member
          Dana Dorsett | | #9


          You don't need a high-temp heat pump- you only need some better focused load reduction, and possibly some radiation enhancement. Before blowing the big money on a heating system it's worth spending smaller but still real number with blower door & IR directed air sealing & remediation of any gaps discovered in the insulation.

          Assuming this house has a full basement (like most homes in Westchester/Rockland/Orange/Putnam county areas) it's likely that the foundation walls of a 1920s house are NOT insulated, and bringing them up to current code minimums would knock something like 10-15KBTU/hr off the whole house load. If it's a stone or rubble foundation insulating the basement walls without creating a mold farm could be a big hunk of change. If the foundation is cinder block, brick or poured concrete it can be DIYed on the cheap using reclaimed roofing foam. (My house was built in 1923 with a poured foundation, which now has 3" of reclaimed fiber faced roofing polyiso on it.)

          If the basement doesn't have a fully finished ceiling, it's also likely to be possible to enhance the radiation on the first floor with plated staple-up radiant floor. In my home there are (circa 1923!) ducts making it awkward to do a full radiant floor up grade to the original part of the house, but in the rooms that matter it's the obvious path forward. Those rooms are currently just fine at design temp with the existing radiation running at an EWT of 125-130F. The ducted zone is currently fed by a hydro-air handler with the same water temps, but the air handler is easily replaced by a heat pump (no-hydronic) air handler.) There is already radiant floor under a 1990s vintage addition (with no easy duct or ductless possibilities), but can easily be enhanced by adding some panel rads to bring the temp requirements down.

          I strongly recommend doing a room-by-room Manual-J using the load calculation of the purpose-made BetterBuiltNW heat pump sizing tool:

          It's a freebie, but you have to share your email to get a free account. Unlike most online tools (or even pro tools), it uses appropriately aggressive U-factor and air leakage assumptions throughout. It also allows you to edit the U-factors for any unusual construction types.

          With the room by room load numbers, look at the ratio of room-load/EDR for EACH room. You are likely to find that many or even most rooms can already get there with <110F water. But you'll know how much radiation upgrade (or load reduction you might want in each room, and work out ways to get there.

          The second floor radiation currently keeps up with 125F EWT with no easy ways to upgrade radiation there, but the upstairs is also not well served by the ludicrously oversized (=ridiculously low duty cycle) central air. But since the kneewall-attic spaces are foam-insulated at the roof deck (ergo inside conditioned space), it's possible to drop in an 3/4 ton compact duct cassette, and with a cold climate compressor it would have more than adequate heating capacity at the local 99% design temp of +5F to heat the space on it's own, as well as sufficient cooling.

          I have considered adding 5" of foam above the roof deck when re-roofing (hopefully by the end of this year), but have been dissuaded due to the complexity & difficulty of mounting solar PV to the roof through that much foam. The rafters on the older part of the house are 20" on center full dimension 2x6,which doesn't offer much insulation space, but the energy savings of adding exterior foam are miniscule compared to the energy production that would garnered from the PV.

          Just a few minutes ago I found out my house has been one of 30 houses (out of 300+ applicants!) to the Mass CEC's Decarbonization Pilot Program. (YAY!) Means-tested additional subsidies in that program won't apply to me (damn- I've been telling mi esposa to quit or even retire- she's working way too hard! :-) ), but we would of course qualify for the standard state & federal programs to mitigate at least some of the wallet pain. Being in the Decarbonization Pilot has other non-subsidy type benefits to energy nerds like me, such as free blower door & IR analysis (and some air sealing correction), and post-upgrades they will monitor & analyze circuit-by-circuit energy use.

          For those in MA looking to electrify everything in their homes, there will be a second cohort selected for the Mass CEC Decarbonization Pilot coming soon- probably before the end of this year. See:

          Слава Україні!

          1. 4lane | | #10

            I re-ran my EDF calc more thoroughly because I have a mix of radiator types which resulted in a different #. What heat emission chart of average water temperature vs. BTU/hr per sqft EDR did you use? Need to see what average water temp provides ~87 btu/hr/EDR sqft.

  4. DC_Contrarian_ | | #11

    Coincidentally, I've been doing similar calculations for my house in Newport County, RI. Design temperature is 9F for heating, 85F for cooling. Doing heat loss by both BeOpt and fuel usage I get a design heat load of 48-50K BTU/hr. The house has an oil-fired boiler that I think is around 150K BTU/hr, 144 feet of finned baseboard radiators and a fan inductor in the bathroom that I think is worth around 6K BTU/hr. Figuring that the baseboards are worth 550 BTU/hr per foot I get 79.2 K BTU from the baseboards plus the inductor gives a total of 85.2K BTU/hr, at a water supply temperature of 180F.

    I had always been told that heat output was linear to the temperature difference between the radiator and the room. With hot water heat it's customary to assume the water drops 20F, so the supply is 180F and the return is 160F, giving an average water temperature of 170F.*

    If I have 85.2K worth of emitters, and I need 50K, I need 58.7% of what I have. With an interior temperature of 70F and an average water temperature of 170F I have a 100F difference. So I can get what I need with a 58.7F difference, or an average water temperature of 128.7. Assuming my heat pump is set for a 10F swing that means water going out at 133.7 and coming back at 123.7. That may be doable, but it seems to be at the upper end of what an air-to-water heat pump can provide.

    One approach would be to keep my oil-fired boiler and only switch it on during the coldest weather. Here's another approach I am considering: I don't have air conditioning. Our design temperature is 85F, we've lived without it. But if I were to install air conditioning it would probably take about 10K BTU/hr. Adding a unit that could also provide an equivalent amount of heating in the winter knocks what I need out of the radiators down to 40K, which drops my water temperature to 116.9F, which I think is quite doable.

    The AC/heat pump could be either a minisplit, or it could be a hydronic air handler that runs off of the same heat pump as the radiators. It would be simpler to have just one outside unit, although minisplits have gotten so cheap, and hydronics is so expensive, that I think the minisplit would probably be cheaper.

    Another approach -- which I may well undertake anyway -- is to improve the insulation of the house and reduce the heating load that way.

    *(Observationally, the water doesn't seem to drop that much. However, the aquastat in the boiler is set to go off at 180F and on at 160F, so 170F is probably a reasonable actual water temperature).

  5. paul_wiedefeld | | #12

    4lane, the one on pg 34 of the Idronics pdf from above. As EDR decreases, water temp needs to increase, all else equal. Luckily, you have an existing boiler to pick up the slack - and/or try to chip away at the heat loss in the neediest rooms. The shape of the total energy needed per year vs. HDD curve is pretty surprising - those coldest days make up a smaller percentage than you’d expect.

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