Heat pumps can be used down to around -15F. This does impose a limit, but it's a lot further north and/or a lot less frequent than most people assume. They sim

Heat pumps can be used down to around -15F. This does impose a limit, but it's a lot further north and/or a lot less frequent than most people assume. They simply periodically pump a bit of heat back out to keep the exchanger free of ice. You'll periodically need a backup source of heat, but most of the time you'll be fine with the heat pump alone.