Q: For homes with existing hydronic (hot water) radiators, what do you recommend? Is it possible to use air-source heat pumps to heat the water on those systems? What about increasing the radiator sizes to operate with Lower water temperatures?
A: Yes you can do this. Daikin, Jaga and SpacePak all make air-sourced heat pumps designed especially for FHW systems. I think it will be a lot cheaper to use the existing radiators and leave the old furnace in place in case the heat pumps can’t keep the place warm in the middle of winter.