I live in the Nordics and my house is prrimarily heated by a geothermal heat pump. The temperature here has been between -20C and -30C for the last week, the pump can easily keep the room temperatures at +19C until the outside temperatures drop under -25C, after that the electric resistor kicks in to provide assistance.

The secondary heat source is the giant baking oven, which i heat every other day if gets this cold. The fully heated oven alone raises the room temperatures by 5-6C, which helps a lot to take the load of the heating pump and keeps the electric bill smaller.

I have been planning to install air/air-water heat pumps to both floors, they would be a good backup during these cold spells and would also provide cooling during the summer. And if I install a smart control system, I can have the air units do extra heating/cooling automatically when the electricity is cheap.

I just haven't had the time to do the research where it would be best to install them and the prices rose steeply after Russia attacked Ukraine. So I'll wait for a while before I'll get those.