 When it is extremely cold outside, it is simpler to heat using an alternative heat source (such as an electric resistance heater, oil furnace, or gas furnace) rather than to run an air-source heat pump.Also, avoiding the use of the heat pump during extremely cold weather translates into less wear on the machine's compressor.The design of the evaporator and condenser heat exchangers is also very important to the overall efficiency of the heat pump.