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Operating a Minisplit Below Grade

casabian | Posted in General Questions on

Hi all,

I have an 18×9 room under my garage that I was planning to have as finished space (perhaps a game room or movie room).

The walls are insulated with closed cell spray foam and I’m planning to close off the space when not in use with a hidden bookshelf.

My questions if I need a mini split in this room and if so, how do people feel about utilizing a mini split that would have to go below grade?

Perhaps it is best as storage but was really hoping to use the space.

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  1. user-2310254 | | #1

    The issue will be finding a unit that can handle such a small heating and cooling load. Do you have an existing ducted system you could tap into?

  2. Expert Member
    PETER G ENGLE PE | | #2

    As far as the original question, there should be no problem installing the inside head of a minisplit below grade. You will need to run the condensate line to a pump or available gravity drain. Make sure the refrigerant lineset penetration is well sealed. Still, even the smallest minisplit will probably be oversized for the space.

    Is it possible to just run a balanced ventilation system with ducts running to/from adjacent conditioned spaces? The temperature control won't be perfect, but this would be a cheap and reasonably effective solution. The heat loads in that space should be very low. If used as a theater, you might find that the equipment and people overheat the space and cooling might be the driving design condition.

  3. casabian | | #3

    Thanks. I can’t easily tie in to existing ductwork. The adjacent room has a return and two vents but neither vent is particularly close to the under garage theatre.

    My biggest question is if I’d have to worry about air quality / mold if I a) close off the space with a door and b) actually use the space to watch a movie etc. when i say theatre it would really just be a big TV and sound bar.

    I’m on Cape cod if that helps or changes anything.


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