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How to site an air source (to water) heat pump on exterior of house


I have been asking a lot of hydronic questions on the radiant side of the forum, but since I am angling to drive my hydronic in-floor system with electricity, I clearly need to increase my knowledge of how an air source heat pump operates. My site is in a cold location (cold and dry, zone 6-7) so I am concerned about the design days and how an air source heat pump will perform. I am hoping to put a unit (hopefully just one) on the east side of the structure but I have a couple questions.

What informs the decision regarding a unit's location on the exterior of a house?

Also, I saw a photo of someone who made a crude "greenhouse" to boost the ambient temperature of the air from which the unit was extracting heat. I am probably not explaining this well, but I have seen other folks discuss this idea as heat pumps start to become more readily used in North America (along with tighter building envelopes). Can someone confirm or refute this possibility? I don't anticipate the need to use the system for cooling and I am absolutely willing to assemble and dismantle something seasonably to improve the COP of a heat pump if such an approach is viable and increases COP.

Thanks in advance. I know there are a lot of folks here with a wealth of design and installation experience and I appreciate hearing what the pros have to say.


  • Jamie Hall
    Jamie Hall Member Posts: 21,523
    I doubt that there is much to be said, one way or another, about where the heat pump is located -- other than to make sure that it does, in fact, have decent air circulation. Your zone doesn't snow much, but... that, and shrubbery and what not.

    As to the greenhouse? Unless it's huge and very well insulated it's not going to make much of a difference.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SWEI
    SWEI Member Posts: 7,356
    If you want to maximize heating capacity, put the outdoor unit where the winter sun falls on it. A passive solar "carport" over the top may help keep the summer sun from driving down the cooling capacity during hot months, and might also help keep snow off the top.

    Enclosing the outdoor unit much more than that would require some careful planning. Be mindful of strong winds blowing directly at the outlet.
  • jb9
    jb9 Member Posts: 104
    Thanks for the helpful suggestions. One thing I should probably clarify is that there is a lot of snow where my site is located, but the ambient humidity is low. There is also a strong prevailing wind from the west/southwest so I am encouraged to hear that I could place the unit on the east side of the house. Is the suggestion about wind mostly one of how it affects the fan and fan bearings rather than air flow?
  • SWEI
    SWEI Member Posts: 7,356
    edited March 2016
    A south orientation will maximize the solar gain. I'd probably face the outlet north if most of your winds come from the south. We have a similar situation here, though the really big winter storms often come from the north.

    The fan has to move air across the coil. A 40 MPH wind (straight on) makes that rather difficult.

    There will be some localized heating of the air in front of the unit from the sun hitting the ground. I'm not sure how much.
  • jb9
    jb9 Member Posts: 104
    Thanks SWEI. So just to confirm, the typically round fan face on the front of most air source heat pumps is blowing OUT/AWAY from the unit and expelling the air from which it has extracted heat. And, additionally, incoming air comes into the unit from the sides, right? I looked at lots of images and saw conflicting diagrams. So if I have prevailing winds coming from the southwest, the south, although ideal for solar gain, might actually not be ideal since it could be fighting the prevailing winds as the system tries to expel the air out the front (round) grate. I am leaning towards the east but I am tempted to construct something with a dark background... My main goal is trying to optimize the site for maximum COP in the dead of winter.
  • BobC
    BobC Member Posts: 5,408
    You could put a wind shield a foot or two in front of the air outlet.

    I just installed a minisplit for the first floor of my house this summer (just south of Boston). The outside unit faces due south but there are no significant winds from the south in my area. So far this winter it's provided about 75% of the heat for that floor.

    I may put a snow shield above my unit in the winter. I won't use in on really cold days but it does a great job when it's in the high 20's and above.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Jamie Hall
    Jamie Hall Member Posts: 21,523
    The COP of an air source heat pump is mostly a function of the air temperature which is being blown across it. So long as it isn't in a location where the wind will really blow at it -- and almost any shelter will do -- the wind won't affect it much. If you do use a wind shield in front of it, though, as @BobC suggests (which isn't a bad idea) just make sure that the wind shield doesn't cause a snow drift right on top of it... :)
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 19,860
    Keep it out from uNder roof overhangs, rain and snow shed can be an issue
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SWEI
    SWEI Member Posts: 7,356
    Fully under the overhang is usually OK, but you have to watch your minimum clearance to the structure wall.

    The kiss of death is locating one on (or immediately adjacent to) the dripline.
  • njtommy
    njtommy Member Posts: 1,105
    Check these systems out from Daikin. I've never seen one in person but would definitely like to. They look really neat.