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Community and Q&A

How to Design Mini Split System for Walkout Basement

jlew1960 | Posted in Energy Efficiency and Durability on

I just came upon your website a few days ago while searching for answers to my question. The more I read, the more I’m finding that I’m not considering all the variables that affect the size, type and location of heads within a given space. Where to start? I have thought about reaching out to a local mechanical engineer to request services but seems that my 900 sf space would be no more than an afterthought.

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  1. Expert Member
    Dana Dorsett | | #1

    >"Where to start?"

    Start with a room by room load calculation (Manual-J or similar). If you don't have pro-software to use, try online freebies/cheapies such as or, and be super aggressive on air tightness assumptions & R-values or they will oversize from reality by a large margin.

    But in most places a code-minimum 900' basement would have both heating & cooling loads well under the output of a 3/4 ton minisplit, unless the walk out side is a long wall of glass sliders or something.

    If there are multiple doored-off spaces with significant loads you'll probably need to go with a mini-duct cassette rather than a high wall coil type.

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