GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Seeking an air-to-water heat pump with small outdoor unit

badmechanic | Posted in General Questions on

We are building a house with a 17′ height limit. We want to put the heat pump on the roof. If we use an outdoor unit above 2’4″ in height, the city won’t let us do it. Has anyone found a short air-to-water heat pump or a way to convert a refrigerant-based heat pump to be air-to-water[A]?

Regarding off-the-shelf short air-to-water equipment, I have found:
1) the Mitsubishi PUHZ-SW50VKA-C200 seems like it would be perfect. However, I have no idea how to import it from New Zealand, where it is marketed
2) Daikin ERLQ018BAVJU is 735mm (29″) but probably cannot be installed in the US.  There is also a July 16, 2022 press release from Daikin launching a 770mm “New Daikin Altherma 3 M” in Europe.
3)  … not much else yet
4) indoor units like the Stiebel Eltron WPL 09 ICS classic. Not exactly what we’re looking for, but I had a hard time finding something like this for a while.

[A] converting air-to-refigerant into air-to-water is discussed in another thread. It really doesn’t seem complicated to hook a plate heat exchanger (with traditional refrigerant on one side and water on the other side) up to any old heat pump, but I can’t tell if people actually do this or if it’s crazy DIY territory that voids warranties and sends HVAC technicians running for the hills.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. DC_Contrarian | | #1

    I think this is going to be a tall order. Compressors generally can only run in one orientation due to lubrication concerns. The trend in compressors is for horizontal fans which makes for an enclosure that's tall and skinny.

    Do you get snow where you are? Even if you found a compressor that was under 28", you'd want to raise it so that snow doesn't block the fan.

  2. badmechanic | | #2

    No snow. Bay area of California. Design low temperature is right around 32 deg F.

    Lots of minisplits have outdoor units that will work. e.g., LMU243HV is 26" high. I plan to ask our mechanical engineers whether they can be made to work as air-to-water. I thought it worth asking this forum since you all are quite well informed.

  3. Expert Member
    Akos | | #3

    Unless you have concrete walls and roof, putting an outdoor unit up there is a bad idea for sound. These are quiet but they do vibrate at low frequency (depending on load it is between 40Hz to 100Hz which readily carries through any stick framed structure. I've run into this many moons ago when trying to mount to a wood wall, the whole place would resonate at certain compressor speeds, it was very quickly removed and put onto to a ground mount. There are ways to mitigate this noise but they are not easy or cheap.

    I would find a spot on the ground and put the unit there. It can always be hidden behind a trellis. Once ground mounted, these are so quiet that you won't hear it run unless you are standing next to it.

    1. badmechanic | | #4

      Thanks for raising that consideration about the noise. The ground is difficult to accomplish because space is very limited. It is not permitted in side or backyard areas and must be 5 feet off the ground due to flood zone requirements. It'd be good to hear if others have similar noise issues with roof mounts.

  4. DC_Contrarian | | #5

    We talked about it in this thread:

    I wish I had links, but I've seen guys do it on the Internet two ways: either a flat-plate heat exchanger or just a copper coil in a tank of water.

    The Chiltrix is a flat-plate design. In order for a flat-plate heat exchanger to work you have to actively balance the medium flow so that the heat flow on both sides is the same. With a variable speed compressor you're varying the speed of the compressor to match the heating demand, then there's a variable speed pump on the water side of the heat exchanger which adjusts the water flow to match the compressor output. The Chiltrix requires a 17-gallon buffer tank inside the house to keep the compressor from short cycling. None of this would be hard to plumb, the challenge is the control electronics to control the speed of the compressor and the variable speed pump.

    The other way I saw (and if memory serves it was a FHB article) was just a 50' coil of 3/8" copper tubing inside a 50-gallon tank. The control of this is simpler because you don't have to balance.

    In either case you're talking about a mad scientist system that only the inventor can operate. For the money you could probably travel to Europe or China and bring home a system.

    I'm sure Akos will be along shortly to tell you that you don't really want hydronic anyway.

    1. Expert Member
      Akos | | #6

      Come now, not always :)

      California is one of the few places that you can get away with heat only as there is no humidity and not much sensible cooling. This likely will change in the next 10 years but for now, heat only can work.

      If you really sharpen your design pencil and simplify the hydronic setup (no buffer tank, no pumps, no/minimal zoning), you might be able to come up with a system that is cost effective.

      Cali is also one of the few places that you can do passivhause levels of insulation for essentially what would be code min up here in the great white north. At that point, the loads are so low that you are heating the place with essentially a hair dryer, so any complicated mechanicals make no sense.

    2. DC_Contrarian | | #9

      OK, this thread has the full discussion of making your own, including both of the guys I had been thinking of:

      It also mentions buying from Alibaba. I took a quick peek over there, and wow, there are a lot of options. You could probably buy two units there for what you'd spend building one yourself.

      1. Tim_O | | #10

        I wonder if these are the same units? I wouldn't be surprised... ArcticHeat comes from China, that's why when you go through the pricing configuration, you get a large tariff added on when importing to the US.

        I've bought things from aliexpress before, it's not too bad. A lot of credit cards let you create a "virtual" credit card number for one time use purchases. But if you charge back a purchase from Alibaba/express you typically get banned from those websites permanently. Regardless of your reasons.

  5. Tim_O | | #7

    The ArticHeat Pool/Spa heat pumps would fit in that sizing. They operate down to -4*F with a decent COP and provide 105* water. It's not going to do all of your DHW, but it gets you most of the way for only $2000. I don't know how well this would work, or how different a pool heat pump really is, just throwing it out there. Might be better than a DIY method, but likely no warranty from the manufacturer if used in this method. They require a larger GPM flow rate- 16GPM vs 6 for the small ArcticHeat EVI pump. I don't know how they would work with either a lower flow rate, or the small tank of water a buffer tank would typically have.

    It also appears that the SANCO2 unit is 26-3/8", so that should fit your bill too.

    1. badmechanic | | #8

      Thanks, Tim. I contacted Arctic. Unfortunately, their smaller heat pump is the older model and isn't recommended for space heating. (It also probably doesn't do cooling AFAIK). The SanCO2 unit is a good call. We can probably use that to replace the hybrid hot water heater we had planned to use and get back more livable space. (But gosh, $5k for a water heater is probably never going to pay for itself vs. more solar + a resistance tank.)

      The Arctic 035ZA/BE model is the shortest air-to-water unit I have found in the US at 34.75".

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |