Advice on Heat Pump Setup for small Two-Story Home with Rental Unit – GreenBuildingAdvisor

I’m in Napa, California, working with local Mits installer.

It’s two-story house with a rental unit downstairs. Part of the downstairs area is below grade, which helps it stay cooler than the upstairs. Currently, the rental gets by with only a small electric wall heater and no active cooling.

The upstairs has a ducted HVAC unit in the attic. I’d like to replace the existing setup with a heat pump located outside the first floor, in the same spot as the current AC compressor.

I’m unsure whether to go with a larger multi-zone heat pump setup or to install a second, separate heat pump to keep the rental unit downstairs completely isolated.

A few years ago, we replaced all the windows with Low-E argon-filled double-pane glass. The first floor is about 725 sq. ft., and the rental’s living space is approximately 497 sq. ft.

I’m considering a few options:

A heat pump with a ducted air handler in the attic for the upstairs, plus a ductless ceiling cassette in the rental’s front room.

For the downstairs rental, either just a single unit in the front room or three units—one each in the kitchen, front room, and bedroom.

Given the way the floor joists run, it might be better to install two smaller heat pumps. The rental’s heat pump could be mounted at the back of the house, with the heating/cooling loop and condensation drain routed through the joist bay to the outside, where the heat pump would be mounted.