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Need help on minisplits for my house

Rick Fetter | Posted in General Questions on

I m building my retirement house in Climate zone 5A at 3360 ft. elevation. It is on a poured-in-place crawl with 3″ of polyiso on the walls and 12 mil vapor barrier.

The single story house has R-42 rockwool walls and R-60 in the attic. I have completed the air sealing and just started the insulation and all the feedback the local HVAC guys are giving me is confusing. One wants to put the ducted minisplits in the attic; another wants it in the crawl space, and another wants to put a head in each of the bedrooms and one in the great room.

The attic is not the way to go but I am unsure of the crawl space. My original thought was I should be able to get by with two units?

I have included my floor plan and hope there is an easy solution. I look forward to any and all information.

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  1. GBA Editor
    Martin Holladay | | #1

    The first step to designing a heating or cooling system is perform heating load and cooling load calculations for your house. For more information, see these articles:

    "Saving Energy With Manual J and Manual D"

    "How to Perform a Heat-Loss Calculation — Part 1"

    "How to Perform a Heat-Loss Calculation — Part 2"

    "Who Can Perform My Load Calculations?"

    You're right that heating equipment and ductwork don't belong in a vented, unconditioned attic. However, it's perfectly acceptable to locate heating equipment and ductwork in a conditioned crawl space.

    It's almost always a mistake to put a separate ductless minisplit head in each bedroom, because a bedroom has a much lower load (in almost all cases) than the minimum output of a ductless minisplit head.

  2. Expert Member
    Dana Dorsett | | #2

    If the crawlspace is inside the pressure and insulation boundary of the house it's fine to put it in the crawlspace. With 3" of polyiso on the crawlspace walls I think you have a winner!

    Two units is probably overkill- run the room by room numbers to find out. A single mini-duct cassette using better-class compressors would likely cover the whole house given your better than code R-values.

    Even in a code-minimum house the "head in every room" approach results in ridiculous and sub-optimal oversizing factors, resulting in lower than advertised efficiency and often with overheating/overcooling comfort issues when it's a multi-zoned compressor. In a high-R house the oversizing factor would be ludicrous, and the efficiency can drop precipitously.

    A tight IRC 2018 code-min 1400-1500' house would have a heat load less than 20,000 BTU/hr @ 0F, and your higher-R house could easily come in under 15,000 BTU/hr at whatever your local 99% outside design temp is. The mini-ducted 1-ton Fujitsu AOU/ ARU12RLFCD delivers 15K @ -5F, and might be a contender. The 1.5 ton AOU/ ARU18RLFCD delivers 18,400 BTU @ -5F. It's unlikely your design heating or cooling loads would be more than that, but run the numbers.

  3. Rick Fetter | | #3

    Thanks for your reply, sorry for the lateness of my thank you as I have been trying to get a manual J done without success. I finally logged into Cool Calc and did their room by room manual J but there are limited options available for above code levels of insulation plus it kept defaulting to tri-cities Tenn. for my climate. This is the first one I have done but I know how to follow directions, sooooooo!
    ..... See attached and let me know what you think. Thanks again for your reply and comments.

    1. Jon R | | #4

      What's up with "Ventilation (none)" in your report?

      Also create a good place (with a drain) for a dehumidifier - you are going to need it to control house humidity.

  4. Rick Fetter | | #5

    There will be a dehumidifier in the crawl space not sure what you mean?

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