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Community and Q&A

Air-To-Water Heat Pump for New House

mikeferro | Posted in General Questions on

Hello, I’m in the process of planning construction of a new home in zone 6 and am leaning toward using an air-to-water heat pump with buffer tank.

It looks like there are a couple tried and tested monoblock air-to-water heat pump options including the Spacepak Solstice SE and Arctic EVI. However, I’m more interested in the split air-to-water options as they do not require glycol and keep the major mechanical components in the conditioned space where they are likely to last longer.

I’ve seen several favorable reviews here and elsewhere of the Nordic ATW Series, but am wondering if anyone has heard of or has experience with the Hydro Solar EVI split-type heat pump (

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  1. user-2310254 | | #1

    Hi Mike,

    I will give your post a bump. With these systems, support is a big challenge. Do you have an installer in your area? Will you have backup heat if the system malfunctions?

    1. mikeferro | | #2

      Hi Steve, the buffer tanks used for most of these units have back-up electric heating elements which are usually sized to cover the load of the house. Obviously the desire is not to use the electric backup, so I'd like to find a system that's reasonably reliable that only relies on the electric backup at the low-temp cutoff.

      The reason I am looking at the air-to-water heat pumps in the first place is because I'd like to micro-zone the bedrooms in the house. Unfortunately, I don't know of any multi-splits that are able to vary head output for this purpose. I know LG and Carrier offer a true VRF system, but I suspect that because these are intended for commercial buildings they are out of my price range.

  2. Jon_R | | #3

    Looking at the box sizes, I don't think the EVI model has the compressor indoors.

    Interesting that Nordic says that expected Canadian average COP might be 2.5 vs 3.7 for geothermal.

    1. AlexPoi | | #4

      The Nordic ATW compressor is in the interior unit.

  3. mikeferro | | #5

    Right, the Nordic ATW is the only one I've found that has the compressor in the interior unit.

    The Hydro Solar EVI only houses the heat exchanger and electronics in their interior unit.

    In either case, the advantage of a split system is that because the refrigerant to water heat exchanger is located in conditioned space you don't need to fill the system with glycol to prevent freezing in cold climates.

  4. Expert Member
    Akos | | #6

    I would rather have the compressor outside and deal with glycol than try to deal with the noise from the heat pump. Most are quiet but not silent. Probably just me, but I have had to replace water pumps in my own home as you could hear them through the piping and rads, I can't imagine leaving with a something like a 2-3 ton compressor in the basement.

    Anything hydronic adds about $20k to your build, a heat pump probably an extra $10k. Make sure there is a good reason for doing it. Hydronic is great for older leaky houses or very large buildings with sprawling floor plans. It doesn't make much sense for efficient housing. The only system I would consider is something like this:

    1. Expert Member
      NICK KEENAN | | #7

      I'm going with hydronic in a house I'm working on. There are lots of rooms and no room for ductwork. I looked at minisplits, but I would have had to put a head in each room to satisfy the local code authorities. Every head would have been ridiculously oversized, the smallest minisplit head I could find was 6,000 BTU.

      I can get hydronic air fan heads that are as small as 3,000 BTU for both heating and cooling. I can derate them further by modulating the water temperature. Just for heating I can go pretty much arbitrarily small. I only need to find room in the walls for a 1/2" pex pipe.

      It's going to be expensive, although in the end I don't think it will be that much more than a ten-head minisplit system, with far superior comfort. And it will be cheaper than ripping the house up to install ductwork, with better esthetics.

      1. charlie_sullivan | | #13

        And if you want less than 3000 BTU, just use a small panel radiator without a fan, and turn it off during cooling season.

    2. AlexPoi | | #11

      20k$ if you are putting radiant floors. Radiant ceilings can be pretty cheap. Radiators and hydronic coil can be a cheap option as well.

      I disagree that it doesn't make sense for efficient housing. Mini split are a good option if you have an open floor plan or if you are willing to leave your doors opens. Otherwise, it's pretty terrible at distributing heat in all the rooms. You need ducts for that and ducts take lot of spaces and are inefficient.

      1. Expert Member
        NICK KEENAN | | #12

        I think a lot of installers are intimidated by radiant floors, because if you mess up the design you're pretty much hosed. And there are lots of ways to mess up the design: underestimate the heating load, underestimate the insulation of the floor, overestimate the thermal conductivity of the floor. And now you're stuck with a system that doesn't heat the house, and the only way to fix it is to rip up all the finished flooring and try again.

        1. Expert Member
          Akos | | #18

          There are two major cost drivers for hydronics. Plumbing bits and pieces add up very quickly, for example a zone typically means about $500 just in parts. I have done ultra budget heated floors and even there just material cost was $5k. The cost of the mini split for cooling is on top of that.

          The 2nd problem is there are very few real hydronic install experts. This means that they can charge a premium even through the work that is needed in a lot of ways is much easer than dealing with ducts. For example, here it costs about the same to install a high velocity AC system (which is not cheap) as it does to replumb a gutjob and move some rads, which is insane.

          You also run into the issue of needing cooling which adds a lot extra complexity to hydronics. Doable with mini air handlers but you have to now be very careful with insulation, condensation on lines and equipment and drains. It does take less space though.

          I have pretty much given up on hydronics. Ducted mini splits are much cheaper, easier to install and higher efficiency. With careful design you can share the ducting for the ventilation with it which you need to install anyways.

  5. Jon_R | | #8

    Hydronic enables optimal comfort with lots of zoning, even for small zones. Like geothermal, it seems to cost much more than seems logical (to me).

    1. Expert Member
      NICK KEENAN | | #9

      Yeah, I don't get it. I'm not a pro but I find pipe easier to run than duct, and water behaves more predictably than air when you pump it around.

      All I can think is that a lot of guys are afraid of hydronic.

  6. joshdurston | | #10

    I grew up in a house with a 4 ton scroll compressor GSHP in the basement and never noticed the compressor noise. The oversized gas furnace blowers in my last couple houses made way more noise.

  7. BirchwoodBill | | #14

    The hydro solar looks to be same as the Artic heat pump. I am in zone 6 and looking at an Artic for a new house design. The thermo2000 tanks have backup heat, so I am not worried about that -30f occasional day, most of the time it is above 5f. To keep the hydronic simple look at a product called warmboard. We remodeled 2 floors with warmboard and keep water temps between 85 and 105. I had to replace the basement baseboard with a low temperature system, but is more comfortable.

    1. Expert Member
      NICK KEENAN | | #22

      I looked at Warmboard but they wanted $240 for a 4x8 sheet, that's $7.50 per square foot. It would have been over $10K just for the Warmboard. It's a slick product but that just seemed beyond practical.

  8. rhl_ | | #15

    SpacePak is about to launch an inverter driven _split_ heat pump

    1. mikeferro | | #16

      Do you have a contact at SpacePak that can provide more info on the new inverter driven split heat pump? Any idea if it's just a Chinese import or US designed/built?

      Reliability is my biggest worry so buying from a known brand who will support their product feels safer than buying from an unknown company.

      1. rhl_ | | #28

        It’s a Chinese import for phnx-e but it does empirically seem to be a better product than a rando Chinese manufacturer.

  9. Jon_R | | #17

    If there is a failure, it's hard to beat a monobloc air->water heat pump for ease of replacement - even with a different brand. Just connect two pipes and wiring (power and control).

    Glycol doesn't strike me as an issue. You probably want something for corrosion anyway.

    1. AlexPoi | | #19

      That's a good point and you don't need a technician to connect the refrigerant lines. But winter can be hard on the compressor.

  10. WalterTS | | #20

    Sounds very interesting, the air to water version of heating/cooling. Does anyone make room radiators(with fans or without) for these ATW units. I wouldn't want just a big air handler and ducts like your typical gas furnace install and radiant floor style would run the cost too high I'm guessing. Like Jon mentioned, they sound easier to repair. I'd be looking at the Nordic ATW.

    1. Expert Member
      NICK KEENAN | | #21

      Do a Google search for "hydronic fan coil unit." These types of units are really common in commercial settings, although a lot of the units are on the large side for residential use.

      Chiltrix, one of the ATW manufacturers, has a couple of good pages on various units at and

      1. WalterTS | | #23

        Yes thanks I went to the Chiltrix site to have a look. Do these work for most ATW units. Now to find some user reviews on the Nordic ATW units.

        1. Expert Member
          NICK KEENAN | | #24

          Chiltrix doesn't manufacture any of the units they list on their website.

          One of the nice things about hydronics is that it's plug and play, fixtures spec a water temperature and a flow rate, if you can meet them you're good to go. Even if you can't meet them it's still going to work, just at lower output.

          1. Expert Member
            NICK KEENAN | | #27

            Money quote from the article:
            "For what it’s worth, published heat pump performance suffers a lot at 5°F and below; the reality is we’ll mostly burn wood when it’s that cold out."

    2. rhl_ | | #29

      There are many. I am installing Jaga convector radiators. They are beautiful and work well. If I did it again I might just add the fan boosters.

      Still choosing my fan coils for cooling.

      1. WalterTS | | #30

        Great, and they're Canadian too. What are the typical prices?

        1. Expert Member
          NICK KEENAN | | #31

          I think they're Belgian but distributed in North America by a Canadian company.

          I went their website. They look great. However, there was no information about how to get them or what they cost, and I read through the "technical information" twice and still can't figure out how to size and select them. Lots of pretty pictures and florid marketing prose though. Everything I hate about manufacturer websites.

          1. WalterTS | | #32

            That makes more sense really that they are European and only distributed by a Canadian company. Yes that doesn't sound good but like you said everything you hate about manufactures websites. Let's see if you have more luck sending an info request via email.

          2. rhl_ | | #33

            The guy Alex Naja who works there gave me prices. I bought the lower end wall units for heating only, and they were like ~300-400/unit. Very inexpensive imho.

            Shipping from belgium (air freight) was a lot.

  11. WalterTS | | #26

    I was wondering if most were just P&P if they matched the temp rating.

    Thanks for the link Alex.

  12. cr0ntab | | #34

    Subscribing because I'm in the middle of an air to water heat pump install on my house and want to see where you end up.

  13. mikeferro | | #35

    I'm re-evaluating specing the Nordic ATW-75 for my new house after noticing that SpacePak is beginning to offer their Solstice split system inverter (SIS-060A4) with cold temp performance down to -22F.

    It sounds like they are stilling putting together submittals and manuals, but I'm hoping maybe somebody here can share more insight about this unit?

    1. jeffrey_miller | | #42

      I'm planning to build a passive house in Bozeman, MT, in climate zone 6B. I've also been looking at air to water heat pumps, and am wondering which unit you ended up using for your project.

  14. rhl_ | | #36

    The spacepak unit is actually this unit imported:

    I'm looking at it myself. I've done some analysis for home my home:

    John Siegenthaler has done a coffee with caleffi on it. He's a great resource.

    1. Expert Member
      NICK KEENAN | | #37

      Interesting, because the hydronic fan units that Chiltrix sells are also rebadged Phnix.

      1. mikeferro | | #38

        I'm guessing that Chiltrix is buying their fan coils from a distributor and reselling them. I talked with Phnix and they have signed an exclusivity agreement with Spacepak parent company Mestek, so they will not sell to anyone else in the USA.

        Frankly, at nearly $10k for the Spacepak split heat pump (SIS-060a) I can't justify using it.

        I'm considering the pros/cons of importing a heat pump from PHNIX competitor Macon ( and using an EcoUltra buffer tank for heating backup.

        1. rhl_ | | #40

          Macon is Arctic Heat Pump. Chilltrix is also a chinese import (they all are)...

          It's too bad the better LG/Mitsu/Daikin air to water avoids the Northeast..

          LG Therma V looks killer.

  15. djmoore14 | | #39

    I almost went with HydroSolar until I found out it doesn't have any CSA or TUV (north american) stickers. Only CE which doesn't mean much here. Not sure what would be required to get this but seems important to watch for. Arctic has gone through the process to get theirs certified for NA. Might be a lot less headache. Otherwise, I think HydroSolars look good. Their rep said they were hoping for TUV this summer but even that won't be the TUV for North America.

  16. Mason_L | | #41

    Enertech has a new AWHP called the Advantage that may be worth looking at.

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