In a recent Q&A post ,“Tanquen” writes that he is working with a local Mitsubishi installer to upgrade his HVAC and needs some advice. He lives in Napa, California, in a two-story house; a rental unit occupies the lower level. The first floor is about 725 sq. ft., and the rental’s living space is approximately 497 sq. ft. The upstairs is served by a ducted HVAC air handler in the attic, which he wants to replace with a heat pump located outside in the same spot as the current AC compressor (see floor plan above). The rental has a small electric wall heater but no air conditioning; part of the lower level is below grade, which helps it stay cooler. A few years ago, all the windows were replaced with low-e argon-filled double-pane glass.

Tanquen isn’t sure whether he should get a larger multi-zone heat pump or have a second heat pump installed for the downstairs rental unit. He lists what he sees as the options:

He could add a heat pump with a ducted air handler in the attic for the upstairs, and a ductless ceiling cassette in the rental’s front room.

Or, he could either add a single unit in the rental’s front room, or three units—one for the kitchen, front room, and bedroom.

He wonders, too, if maybe a separate heat pump for each floor makes the most sense, as the top floor will be ducted and hotter than the bottom floor, which is below grade on one side, stays much cooler, and may be unoccupied at times.

Last, he considers one large 9000-18000 BTU/H cassette or three small 6000 BTU/H cassettes for the rental space, reasoning that three would achieve the desired temperatures faster and operate continually.

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