When we yanked the oil boiler we replaced it with a wall-mounted minisplit heat pump in the main level open area that includes kitchen, dining, living and our little office area. We closed off the first floor bedroom and bathroom so those rooms are only heated by conduction and air leakage through the walls, and so they get cold - in the high 40Fs at the lowest last winter. The second floor bedrooms and bathroom are only heated by warm air from the main floor rising up as cooler air from upstairs drops. This is a natural convection cycle, and it works pretty good and for free! Doncha love physics?
I'm datalogging temperatures in the basement, main level, upstairs, and outdoors with Hobo dataloggers. Here's data from the past week:
Outdoor temp dropped as low as about 16F and climbed as high as 57F. The upstairs ran 2-4F cooler than the main level, except when it was good and sunny and it heated up with more unshaded south glazing per sf of floor area than the main level. Dec 26th was the only good sunny day in the six days shown in the graph and you can see where the upstairs temp climbs above the main floor temp briefly. We set the heat pump thermostat to 70F when we're here and to 66f when we are asleep or gone. This being christmas week we've been here a good bit more than usual, since both of us have been off of work since the end of day on the 23rd, which is the first full day of this series shown above.
If one of us were to be working at home upstairs, we'd need some supplemental heat, probably an electric radiant panel, like the ones we put into the Eliakim's Way homes I wrote about here. A panel like this heats up quickly and can be located so that it heats a person in a fixed location, like a desk, directly. It's a good supplement in a point source heat pump house.
Of course, we could also install more heat pump cassettes. In the most extensive case, there would be four, the one we have and one in each of the three bedrooms. I think if we ever use the first floor bedroom we might install another Fujitsu single zone unit for that room, albeit the 9,000 BTU/hour model rather than the 12,000 BTU/hour unit we have now, because it's a smaller space. Then we'd add the radiant panels in the upstairs bedrooms as well, which we may do at some point as I have two of them laying around in the basement! But for our present use of the house, the point source system works well, and was inexpensive way to get off of fossil fuels and let the PV system supply the energy we use for heating (albeit on an annual basis...)