We decided it’s time to work towards complete electrification of our house. It’s half a duplex in NH 03861, 2200 sqft, unconditioned (for the time being) walk-out basement. It’s built in 2006, 2×6 R-19, double-pane vinyl windows, not particularly high build quality for all I can tell. I DIY’s air sealing in the attic some years ago.

TL;DR version: Plan is to go for 1-1 downstairs unit first (ceiling cassette or wall unit?), then make a plan for upstairs as the next phase (ducted minisplit?)

Our current heating is forced air propane (110k BTU input, 95%). I’ve calculated heat loss a number of different ways: I picked a design temp of 7F (averaging some data sources). I calculated design loss from our 2-year consumption of 570 gallons/year to be 21.5 kBTU/h. Using loadcalc.net, I got 28.5 kBTU/h, and doing my own manual J, I ended up at 27.6 kBTU/h, with some assumptions about typical winter basement and garage temps.

The one datapoint that doesn’t quite fit in there is that I think I remember looking at the Nest thermostat in the polar vortex thing this February, where it got down to like -15F, and it seemed like the furnace was running half of the time, though it’s also hard to tell because we have two zones, so there’s overlap. Anyway, since the other calcs are kinda in the same ballpark, I’m inclined to disregard whatever I remember from Februrary and it looks my furnace is like 4x oversized… 

My initial plan (when all I had was the furnace running half the time, ie., 50k BTU/h data) was to do three Mitsubishi 12k ceiling cassettes in the upstairs bedrooms with the 36k outdoor multi-split unit — and I figured if that’s too much, I could add a ceiling cassette downstairs on the same unit. Well, fortunately I started doing some actual research (in particular GBA was very helpful), and clearly ceiling cassettes upstairs aren’t a good match for what turns out to be 3-3.5k loads for each of the three bedrooms. So I think this does look like a case for a ducted minisplit, but that’s also somewhat iffy since really the only feasible place is the unconditioned attic, and I might consider building some kind of enclosure there, but that’s clearly a bigger project.

Since I also learned that multi-splits aren’t necessarily a great fit due to limited turndown, I figured I’ll turn my plan upside down and start downstairs rather than upstairs, and may use next winter to figure how much additional capacity I need upstairs (the propane furnace will still be there to help, though I think for the shoulder seasons maybe enough heat will rise from downstairs). So I’m now thinking to do a phase 1 with a single 1-1 9k or 12k indoor unit downstairs (which actually has a heating capacity of 12k/15k down to 5F). 

Does this sounds reasonable? A particular decision I’m struggling with is whether to get the one-way ceiling cassette (Mitsubishi MLZ/SUZ) or the regular wall unit (MSZ-FS/MUZ-FS). We really would prefer the in-ceiling variant (mostly for aesthetic reasons) but I’m concerned that the much lower turndown of the MLZ/SUZ might significantly reduce efficiency. Any opinion/experience with that?