 Since ASHPs have high capital costs, and efficiency drops as temperature decreases, it is generally not cost-effective to size a system for the coldest possible temperature scenario, even if an ASHP could meet the entire heat requirement at the coldest temperatures expected.Propane, natural gas, oil or pellet fuel furnaces can provide this supplementary heat.All-electric heat pump systems have an electric furnace or electric resistance heat, or strip heat, which typically consists of rows of electric coils that heat up.