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I have a question about sizing ductless for cooling

vernacubuild | Posted in Mechanicals on

I’m building a 1,043 sq ft, 1 and 1/2 story cottage. I had independent heating and cooling load calculations done. I’m going with two Mitsubishi Hyper heats, one up and one down.

For downstairs, the cooling load is 13,778 BTUh and the heating load is 1,1524 BTUh. The energy consultant says his calculations are conservative, for the strict purpose of not oversizing. I would like to go with the 12K BTU Hyperheat for downstairs, but it’s only rated at 12K for cooling. Would the slightly undersized 12K be OK for this application? The downstairs includes living area, kitchen, bath and a bedroom in 630 sq. ft.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Would the slightly undersized 12K be OK for this application?"

    A. Probably, but there are several factors to consider.

    First of all, a single ductless minisplit serving 4 rooms will not provide perfect distribution of heat or cooling. For some occupants, this is just fine; others might complain. The set-up works best if the occupants usually keep their doors open.

    Second, a room with west-facing windows can overheat on summer afternoons, so if such a room is far away from the minisplit, you may have a comfort problem.

    Finally, everybody has different comfort expectations. Some people don't mind a few degrees of temperature difference from one room to another; others do.

  2. vernacubuild | | #2

    Thanks Martin. The only west facing glazing downstairs is shaded by an 8' deep porch roof. The upstairs unit serves two bedrooms totaling 413 sq ft. It will be placed at the top of the stair landing. The west facing room is at the bottom of the stairs. Is it reasonable for me to assume that unit would help cool that room? If not, do you recommend going with the 18K?

  3. GBA Editor
    Martin Holladay | | #3

    I think the performance results will have more to do with distribution issues and occupant flexibility than with the sizing of the unit.

  4. vernacubuild | | #4

    Thanks Martin. I'm meeting with the HVAC contractor next week. This is my personal home (plan to replicate as spec soon) so we won't have a problem with keeping doors open. I just need to determine the best placement for the head units at this point.

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