Retrofit single ductless minisplit to cool entire upstairs
Hello, I’m a huge fan of this website, but I finally joined so that I could ask this specific question:
My wife and I own a 2000 sq-ft colonial built in 1962 in the Metro Detroit area (climate zone 5), with only baseboard heating–no central air. The first floor can be kept comfortable with a single wall-mounted AC unit and some fans to help circulate, but the upstairs is a bigger challenge. We’ve had several options quoted to provide some form of AC, but we’re leaning strongly towards a ductless minisplit system.
However, the problem is that I don’t know what arrangement makes the most sense for the upstairs. The main challenge is the layout; it’s essentially a 16 foot hallway with 4 bedrooms off of it. Originally we were quoted for a 4-zone system, with a head in each bedroom. However, after further reading, it seems that design would be overkill… especially after getting a cooling load calculation of roughly 10,000 Btu/hr for the entire upstairs! (Though even that # may be high… the heating load calc they provided was about 70% higher than reality!)
I’m wondering, could get away with installing a single 9K Btu unit in the hallway, and have it service the entire floor? There are several articles that talk about using a single unit per floor, but they’re regarding “tight” houses. Our house is likely drafty by GBA standards, but last winter we had a major sealing and insulation job done on the house… basically made the attic, basement rim joists, and overhangs air-tight, and put cellulose insulation (R-49 worth) in the attic. The CFM-50 went from 3645 to 2465.
So the question is, how well would a single 9K minisplit be able to cover the upstairs of our house? My intention would be to leave it running all summer, in order to keep the whole floor roughly 72-75F and dehumidified. Also, we would leave bedroom doors open during the day. My hope is that we wouldn’t need additional fans to push the air into bedrooms. Would there be enough cooling through convection and the internal walls to combat the heat entering through the external walls & windows? Each bedroom has 2 small double-paned windows, and we get enough shade to prevent much direct sunlight, except for early morning and late evening.
Thanks in advance, and sorry for the long post!