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Community and Q&A

Sizing a Minisplit for a Terribly Built Condo

Melissa Ames | Posted in Mechanicals on

A client is considering a mini split for her zone 5b, 1600sf, 3 room condo. It was built in the early  seventies, and built badly. This particular condo is ground floor, an end unit, one story, three walls are exterior. The roof is 2×12 rafters, walls are 2×6 on 24″ centers. The sheathing is some kind of cardboard, about 1/8″ thick. Very bad installation of batt insulation throughout. Building plans have disappeared, and I’m wondering how to size equipment. It’s also very air-leaky. Originally, the condos had ceiling electric radiant. In units where the radiant still works, the bill runs around $400/month for identical units between November 1st and April 15. It seems like a worst case scenario, based on BTU/sf might be almost as accurate as a guess filled manual J. What do you think?

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  1. Brian Wiley | | #1

    Hi Melissa, one thing you might try is to determine the heating requirements based on the previous bills for when that unit's heat was working. The method outlined by Dana Dorsett works really well. I had a Manual J done, and also did his method, and they were within just a couple of percentage points of what was required. It can be found here:

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