Advice on Heat Pump Setup for small Two-Story Home with Rental Unit
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.
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The best comfort is one air handler per compressor (outdoor unit). I'd be looking at two compressors and two ducted air handlers. In the basement at least it looks like it would be straightforward to put an air handler in the utility room and run ducts to each room.
The downstairs is under 8 ft. That's why I'm trying to use the ceiling cassettes. Everything already feels a little small and cramped, the last thing I want to do is add a bunch of boxes and conduit inside that space. I did think about trying to use an air handler but that would just eat up too much of the room in the utility room, for the air handler unit itself and the ducting.
As I said, it's been getting by for like 40 years with a rinky dink electric wall heater. It seems adding almost anything else would be better. Not to mention the fact they would now be able to heat and cool.
Tanquen,
Can you use ceiling cassettes between the units? The ceiling is a required fire separation.
Good question, no one's brought it up but I'd wonder if recessed lighting and the like were also against code. I believe the cassettes are self-contained and would not allow fire directly into the joist bay. ???
The common way here for ADUs is to have a wall mount in the living space on its own outdoor. Resistance heaters for the bed and bath. This gives you cooling and the bulk of the space heat without having to worry about multiple units. This way also if the ADU is sub metered, all heating costs are captured. I guess it could be ceiling mount as well but those tend to be harder install.
For the main area, 3 units for a 500sqft are in California is well beyond overkill. The loads there will be so low that the smallest 3 zone unit will short cycle all the time.
I would say you best option is a larger head in the living space and the ducted unit upstairs.
A better option is to add some additional ducts from the upstairs unit down to the main floor and have just a single ducted unit for the whole place. The SVZ multi position air handlers can do a fair bit of static pressure so with reasonable ducting you should be able to get air to everywhere.
There are three openings, I'd say conduit but they're not enclosed. They're just access ways where I believe they had exhaust for the gas water heater and possibly some sort of boiler originally downstairs. I was a planning to use one of those for the heating/cooling and condensate lines into the attic.
Not big enough to bring down ducted air from the air handler in the Attic. Two stories above. There would also be the issue of the first floor possibly, not wanting much if any cooling and wanting heating on the upstairs wants cooling.
I speed read your description. Looks like the place is only two stories total. In that case I would go for ADU option above and the single ducted unit for the 2nd floor.
You are also in the land where you can have an unvented attic with just fluffy in the rafter bays as long as you install a diffusion ridge vent. This might be a good next step in energy improvement since you have the air handler up there.