All heat pumps are generally still efficient at well below freezing, and will have a COP (efficiency) above 1 even at -15C. The problem is that they don't gener

All heat pumps are generally still efficient at well below freezing, and will have a COP (efficiency) above 1 even at -15C. The problem is that they don't generate a huge amount of heat at those ambient temperatures, so you need a backup source of heat. A heat pump should be able to keep your house at room temperature when the outside temp is around freezing, but it will take a long time to heat up your house if you let the temperature drop overnight. 40F seems unusually high for your aux heat set point, unless you have expensive electricity and cheap gas.