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Minisplit sizing for a cold climate

tightwad | Posted in General Questions on

Got a preliminary quote for a Mitsubishi hyper heat mini split, two heads. The manual J numbers are forthcoming.
The downstairs (heating) head will be 18,000 BTU. The 2nd story head, 9,000 BTU, is for a/c, which we only need when there are forest fires and we can’t open windows at night. A heater isn’t needed in the 2nd story, so the 18,000 BTU head downstairs will be the only head providing heat. The outdoor unit will be 24,000 BTU.
We have backup baseboard heat for our 2 cold months, when nighttime temps range from 0 to -20. I don’t expect the mini split to provide enough heat in the coldest weather, but I hope it will take care of the other 6 months of heating.
The house is 1400 sq ft with a cathedral ceiling throughout, with one upstairs bedroom. Our climate is 5000+ degree days. We use heat from October 1 to May 30.
Will this design meet my expectations? Does it make sense to heat the house with an 18,000 BTU head and a 24,000 BTU outdoor unit? I fear a larger unit, which the manual J will probably call for, will not be efficient in our coldest weather, and will be oversized for our long shoulder seasons. For example, our low temp tonight, July 1, will probably be in the high 30s.
Thanks for your thoughts,

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  1. user-2310254 | | #1


    Please post the Manual J when you receive it. GBA's general advice is to get this done by an independent engineer since HVAC contractors tend to produce reports that support their general rule-of-thumb for sizing systems. (See

  2. Expert Member
    Dana Dorsett | | #2

    >"...our 2 cold months, when nighttime temps range from 0 to -20."

    Is that Fahrenheit or Celcius? (I'm assuming Celcius)

    >"Will this design meet my expectations? Does it make sense to heat the house with an 18,000 BTU head and a 24,000 BTU outdoor unit?"

    Without knowing the design load or the expectations it's hard to say. A 2 ton ductless with a cold climate type vapor injection compressor can deliver about 24-25,000 BTU/hr @ -20C. A head rated for 18000 BTU/hr in cooling mode is usually good for about 22-23,000 BTU/hr in heating mode if there is enough compressor behind it (which there would be, if it's 24,000 BTU/hr COLD CLIMATE multi-split.)

    That is enough to heat cover the typical design loads of a tight, insulated 1400 square foot 2x4 framed house with clear glass (no low-E) double panes, or an even bigger 2x6 framed insulated house with low-E windows. Distribution of that heat can be a bit uneven, with a few doored off rooms probably getting too cold for comfort at -20C, unless its a VERY well insulated house. If there is a basement or conditioned crawlspace for routing ducts, a ducted 18K mini-duct cassette can be a better solution than a high wall type ductless head.

  3. mcmodern | | #3

    I would look at using separate mini split units here

  4. Jon_R | | #4

    Size per a proper Manual J/S. Partial-load performance will be fine. If you really want to, under-size it slightly and use resistance heat to assist on the coldest days.

    +1 on thinking about adequate distribution.

  5. _Stephen_ | | #5

    The design heat load for my larger house, in a colder climates is only 18k BTU. We can't know if your system is reasonable until we know what your loads are.

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