My 2 cents on mini-split Air Source Heat Pumps (ASHPs) for heating is:
1) if you calculate the source energy usage, it’s pretty much equiv to using nat gas directly because of the efficiency of power plants being ~33%. So 33% * whatever COP you get from thre ASHP (let’s say 3 as a seasonal winter avg) then you are back to ~1.00. Same as using a 95% efficient gas furnance or boiler in terms of primary energy/source energy.
2) Money wise, you’d also have to do the math with electricity vs gas prices. Because even if the energy use is equivalent, perhaps the price of electricity vs gas/oil is mismatched.
3) And of course, if you source the electricity for the ASHP from wind/etc then that’s better too.
4) But one could also skip the ASHP and just buy New England Wind Fund credits to offset heating energy too.
5) Comfort wise: it’s point-source heat, so if your house doesn’t already have heating zones, then putting one where you spend a lot of time would allow you to drop the temp in the rest of the house.
6) But they are slow to heat up a space, so you can’t really use set-backs you can like with furnaces/boilers that blast! Maybe they wouldn’t work that great for point source heat since it’s not like a hot woodstove.
7) The compressor makes noise outside and you need somewhere to put the thing and shovel snow away from it occasionally
8) Another thing to break
9) Holistically: In addition to Wind Fund offsets, one could also insulate the house more, eat less meat, drive a prius or more efficient car, or move closer to work so you can walk, bike, or drive less. Or fly less.
10) ASHP will also give you AC. So that might be nice.