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Community and Q&A

New Air to Water Heat Pumps on the Horizon

nickdefabrizio | Posted in Mechanicals on

I have an oil boiler/hot water baseboard system as the initial heat source.  I then installed two 12kBTU Fujitsu low temp mini splits in the main part of the house and tonight (9F) they are doing fine. I am planning to finalize the switch to all electric soon. However, I must admit I like baseboard heat a little bit more than hot air heat.  It made me think about Air to Water (A2W) heat pumps.

As most of you know, the problem with A2W systems to date is that the water temps they produce (e.g., 110-120F) will not work well with existing residential baseboard systems designed for 180F boilers. But recently I was speaking with The Radiant Store in Troy NY and they mentioned that there are new systems rumored to be out in the near future that may be high output. Then today I  saw an article about an A2W heat pump developed by Feenstra/VattenFall in the EU that is purported to deliver 160-180 water using C02 and a big buffer tank.

Obviously a plug and play A2W unit that works with my existing baseboard would be a game changer for me and many others who have baseboard. I am wondering if anyone has any further knowledge about this system (or other A2W systems that might be on the horizon) and when  we might see interesting new developments in this area….either in high temp units or some other advancement in these systems…

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  1. Expert Member
    NICK KEENAN | | #1

    Interestingly enough, this was in my in-box today:
    "f you were interested in SpacePak but were concerned about whether the units would work in your colder climate, we've got good news for you: The new Solstice Split Inverter system is able to provide both heating and cooling in climates that get as low as -20f."

    The link is:

    I know nothing about it, just forwarding what I received. It does sound interesting.

    1. nickdefabrizio | | #2

      Yes thanks. These seem to be nice units that probably borrow technology from Air to Air low temp split systems; then marry it to a water heating unit in a monobloc design. I think because they are R-410 based, they top out around 120F, effectively eliminating the plug and play aspect for existing baseboard systems designed for 160-180F water temps. The online seminar they offered in February seems interesting and may portend future models with higher working temps, however.

      For folks designing new systems, the 120F systems seem to work well with radiant in floor, low temp radiators, FCU's or even high efficiency baseboard like the Smith H-2 models that put out almost 400 btu's per foot at 120F. They also provide cooling through FCU's. Certainly something to consider along with or in competition with forced air systems. It seems hard to compare ultimate efficiency on some of these though.

  2. charlie_sullivan | | #3

    What I did was to add some panel radiators to supplement my baseboards and run them all at low temperature. Combined with some envelope improvements that's worked great--I rarely have to raise my water temperature above about 95 F. You can also consider floor or ceiling radiant heating where that's easy to add for effective heating without high temperatures.

  3. johnnydc | | #4

    Hi Nick, Thanks for letting us know about this. It is absolutely what I am looking for as well, albeit full radiators, not baseboard heat. I wish it were here already, and from what they are saying it will 'be rolled out later this year' meaning it is not present even in the Netherlands, much less the UK which they see as their next market. Somebody has to be planning one of these for the US market?! I'm not urgently in the market for this right now, as the old gas boiler continues to plod along, but wow, I am ready for this! Link below to the announcement from Vattenfall.

    1. charlie_sullivan | | #5

      There are some that can get you to 130-140 F, which might be enough if you also do some improvement of the envelope, and maybe adding a few extra emitters. For example,

      1. johnnydc | | #7

        Thank you! This looks interesting, sure enough.

  4. seanm10660 | | #6

    I also would love to swap out my aging gas boiler for an air-to-water heat pump. I've had my eye on the European R32-based A2W systems like the LG Therma V. These seem to be able to generate higher water temperatures at lower air temperatures, compared to their R410 systems. LG's newest "Monobloc S" can make 130F water down to 5F maintaining 95% rated capacity, which seems pretty good.
    Most of my baseboard is oversized for the current heat load, and I've calculated that I can lower my water temperature pretty far (in most rooms), though 130F at my coldest outdoor temperatures (design ~8F) might be tight.
    I'd love to see more of these C02 based heat pumps enter the market, though!

  5. genedj | | #8

    High temperature models have been announced in Europe from Viessmann, Bosch, Mitsubishi and Feenstra/Vattenfall, and others, I believe. But none of these companies have announced plans to bring them to the US. The European market for hydronic heat is larger than ours which is mostly in older northeastern cities. Most of the US uses air-base heating and cooling. With the gas shortage in Europe due to the war in Ukraine, they are trying hard to replace boilers with heat pumps.

  6. LukeInClimateZone7 | | #9

    The challenge for hydronic heating systems is the return water temperatures-- it decimates the efficiency, especially of CO2 based systems. I've heard of sanden units used for heating applications but historically that has been supported or recommended. I think they've evolved some controls to improve performance, but it's still a challenge thermodynamically.
    The mantra at the hot water forums few weeks ago- "AWHPs are NOT BOILERS"

  7. jberks | | #10

    Just my curiosity, what is the issue with running a lower temp for longer Or continuously?

    To my understanding high temp systems essentially need thermostats to start/stop and prevent from overheating.

    I'm in Germany at the moment, and most systems are radiant panels without T-stats, but have a flow metering valve at each panel and you dial it in each room manually. (Just the bedrooms since the high NG prices) And it seems the main circulator is just running continuously. Unfortunately I didn't bring my temp laser with me, but by touching the incoming pipes I think the water is a lower temp like less than 120F. Although, they're not very reactive, like it can take several hours to stabilize a bedroom (by feel) if it's cold and the radiator was completely off.



    1. wastl | | #11

      Hi Jamie,
      it depends .. You can use T-stats even in cold temp. systems to control overheating due to "non - heating energy "(sun, appliances) coming in. Even then, you have outdoor reset and try to keep the supply temp. as cool as possible, if you accept some temp. swing over the day then you can omit the T-stat and try to do all just with the volume flow to each zone/room.
      (It's a code thing here in Germany- T-stats are normally required)
      As an example, I use column rads here and typically run the supply below 90F for ambient temp. of 40F.

      edit - I guess the cold temp. are an attempt to be as efficient as possible for condensing boilers or heat pumps. Properly insulated buildings you try to heat 24/7 with lowest possible supply temp. (Nerds-way of heating..)

    2. Expert Member
      DCcontrarian | | #12

      The issue is you may not be able to provide enough heat.

      The heat output of a radiator is proportional to the difference in temperature between the radiator and the room. So if room temperature is 70F, a radiator at 170F is 100F away and a radiator at 120F is 50F away. The 170F one will output twice as much heat as the 120F one. If the radiator is sized to meet the need for heat on the coldest days with 170F water, it may not be able to keep up with 120F water.

      Typically though, radiators were oversized, because they didn't have good sizing tools and there's not much of a performance penalty to oversizing. And the heating load can be reduced by adding insulation and sealing leaks. So it may be possible to use existing radiators with cooler water.

  8. plc359 | | #13

    There are a number of brands that make air to water heat pumps in the US, such as TACO or Entertech, but none of these products are suitable for baseboard heating. They could be used with radiant floors that are designed for low temperature hot water or domestic hot water, at least they claim. I would steer clear of any CO2 heat pumps, as finding parts or anyone qualified to service these systems will be few and far between.

    Most of the air source options available will use liquid injection compressors, which helps to extend the heat pump operating range to lower temperatures. A traditional compressor can handle about 90 degrees of lift, for instance from 30 degree outside air temperature to 120 degree hot water temperature. A liquid injection compressor can handle about 120 degrees of lift, which is how a cold climate air source heat pumps can operate below zero degrees and supply 90 degree air relatively comfortably. It should be noted though, that efficiency suffers greatly for this extra lift, the system Coefficient of Performance may drop below 2 under heavy lift situations. Trying to lift from below zero to 120 hot water may theoretically be possible, but the longevity and efficiency will suffer so greatly that you'd be better off with electric resistance. I've heard of these compressors dying in less than one year when attempting to exceed their lifting limits.

    Then you have a second problem, which is that 120 degree hot water is not hot enough to keep your house warm on a cold day. There is someone in the Albany area though who has figured out the best solution to this problem, probably the same group that your Radiant store contact knows. The company is called Energy Catalyst and they build a geothermal Double Hybrid heat pump that is designed specifically to reuse baseboards/radiators. Their system has very high efficiency and they have units all over the Capital Region. I went to one of their Open Houses this past winter and they could get the house very warm on a cold day, the homeowners seemed to love it. My wife and I are looking to use them for a church retrofit that has baseboard heaters. I'd recommend giving them a call.

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