An air-source heatpump is the most common type of heat pump. The heat pump absorbs heat from the outside air and transfers the heat to the space to be heated inthe heating mode. In the cooling mode the heat pump absorbs heat from the space to be cooled and rejects the heat to the outside air. In the heating mode when the outside air approaches 32o F or less, air-source heat pumps loose efficiency and generally require a back-up (resistance) heating system.