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Vertical stacking of heat pump outdoor units

this_page_left_blank | Posted in General Questions on

I’m in the process of finalizing the outdoor placement of a heat pump. This may end up being the only one, but there’s also a possibility we’ll add one for the second story next year, based observations we make this summer.

The ideal placement, laterally, is the same for both the imminent install and the hypothetical second unit. I’m thinking that I could have the second unit installed above the first, with the drain directed around the lower one. Is this a stupid idea?

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  1. Expert Member
    BILL WICHERS | | #1

    It depends on the design of the particular outdoor unit you're using. You need to keep them out of each other's airflow so that one unit doesn't suck in exhaust air from the other. Condensate drains are much easier to route than air paths, so I wouldn't worry about that as much.

    Basically if the unit exhausts out the top, I'd not stack it. If it exhausts out the side, it might be stackable, with a bit of separation. I'm assuming you have only about one story worth of vertical space to fit these. I usually work with really big units (commercial size), and the manufacturers for those generally specify a minimum separation distance for the units to avoid too much interaction (and resulting reduced capacity).


  2. kelchm | | #2

    Look for the installation manual of the particular outdoor unit(s) you are considering. It should give you specific min distances to all surroundings including adjacent/stacked units.


  3. this_page_left_blank | | #3

    The manual says 24" clearance above. The upper one would only run in the summer, and the shared load would be well below rated capacity.

    1. Expert Member
      BILL WICHERS | | #4

      Then there’s your answer. I’d treat that 24” as a minimum, and separate the units more if you can.

      If the lower unit raises the effective ambient temperature for the upper unit, you’ll loose some efficiency on the upper unit. Sometimes a baffle can help direct airflow from one unit away from the other, so that might also be something to think about.


  4. FluxCapacitor | | #5

    Seems like an OK idea. There are companies that make stands specifically for this:

    Biggest issue I see would be securing the rack from tipping and getting the lower unit above snow levels (if needed).

    There’s also the option of hanging on the wall, although my installer advised against it due to clearance and noise issues.

    1. this_page_left_blank | | #6

      Mine is being hung on the wall. I'm assuming they will follow the minimum clearance specs, and my walls are almost two feet thick so I'm not too worried that much sound is going to make it through.

      1. Expert Member
        BILL WICHERS | | #8

        You can mount the unit with vibration isolators similar to the one in this pic to help to decouple it from the wall acoustically. Note that these need to be installed vertically like the pic, they don’t work as well in shear (horizontal). I’ve used these before to keep pump motors and the like from coupling sound into walls and other structural elements. They are surprisingly effective for how cheap they are.


  5. AndrisSkulte | | #7

    If it's cold climate, make sure the top unit won't drip and freeze into the bottom unit. Mitsubishi has a cold climate installation guide and tells you not to stack in that scenario.

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