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Community and Q&A

Stacked Minisplit Outdoor Unit Freeze Up Risk in Cold Climates

j_prescott | Posted in Mechanicals on

I have some limitations on placement of two outdoor units for ducted mini-splits. I’m planning to place them one above the other on a stacked stand.

However, I’m located in climate zone 6 and concerned about the risk of condensation from the top unit accumulating on the lower unit and causing excessive defrost cycles or icing and freeze up.

The units have pan heaters and adding a pipe from the upper unit’s pan to drain past the lower unit might be an option, but then again, maybe the pipe would just freeze too.

Some photos from the web are attached illustrating the type of stacked stand I might use and ice accumulating under an outdoor unit in a cold climate, which illustrates the concern.

Has anyone else run into this problem? 
How, other than not stacking units above each other, did you solve it?


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  1. Expert Member
    Akos | | #1

    I've avoid the problem by not stacking units, in a cold climate most mini splits will make a pretty nice ice buildup underneath.

    I did have one outdoor unit that was getting a bit of roof melt onto it, it tended to run in defrost a lot trying to clear both the water from above and frost buildup on the coil. In the end, I ended up installing a mini roof above the unit which really helped.

    If I was stacking units, I would put a fair bit of space and include a mini roof between units. Most also have an option for a fitting for drain connection on the pan, the problem is that you have to deal with heat rope (not cheap to run and can still fail) to keep it from freezing solid.

  2. mgensler | | #2

    Quicksling makes minisplit weather caps as does a sheet metal shop in Maine. The ice forms on the front instead of getting in the coils. We have one from the Maine shop. It works well, the fit and color match look OEM. Highly recommend

  3. Expert Member
    KOHTA UENO | | #3

    Robb Aldrich from SWA did a great presentation at the Passive House conference on MSHPs, including potential risks of "stacking" the outdoor units vertically (slides 10-13). However, he showed a very elegant solution of piping with giant drain pipe to catch the condensate and get it away from lower units--see slide 14.

    Overall, stacked minisplits are a similar idea to a two story outhouse. :D

    1. Patrick_OSullivan | | #5

      Unrelated to the freezing issue, something from that deck caught my eye:

      - Low fan speed cripples ductless capacity and efficiency.
      - Study in test homes found forcing ductless HP in HIGH speed increased COP by 60%"

      A naive explanation of this is that there will be higher relative parasitic losses at lower speeds. But, should this be interpreted as saying that a shorter duty cycle at high output is preferable to a longer duty cycle at low output?

  4. Expert Member
    BILL WICHERS | | #4

    My preference would be to not stack the units, but if you don't have any choice, consider having a sheet metal shop fabricate a stainless steel pan with a drip edge for you to place under the top unit. What you want is a pan with a slight upward facing lip on the two long edges that will channel any runoff to the ends of the pan, and a bevel on the ends to drain any runoff to the sides of the units and not into the air intake or exhaust grills. I'd have them make this pan out of 304 stainless of around 0.04"-0.05" or so thickness for reasonable strength, but without going too overboard. Have them punch holes for mounting bolts so that you don't have to deal with drilling metal sheet in the field.


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