A booster unit and method increase the performance of an air source heat pump system at low ambient air temperatures, the air source heat pump system including a conduit system for forwarding a refrigerant through an external circuit exposed to ambient air. A tubular system is immersed in a liquid heat exchange medium, such as water or antifreeze, within a booster chamber having chamber walls exposure to ambient air. An internal circuit of the tubular system receives refrigerant from the conduit system for advancement through the tubular system and delivery back to the conduit system so that heat passing from ambient air through the chamber walls and into the liquid heat exchange medium in the booster chamber is transferred from the liquid heat exchange medium to the refrigerant in the tubular system, to increase the temperature of the refrigerant being delivered from the tubular system and forwarded to the external circuit, thereby reducing or eliminating frosting at the external circuit.