Abstract: An air-to-air heat pump has a first passageway having an inlet and an outlet both communicating with air outside the building, and having a first heat exchanger constituting an evaporator for a first refrigerant contained therein during a heating cycle, and has a second passageway having an inlet and an outlet both communicating with relatively warm air inside the building, and having a second heat exchanger constituting a condenser for the first refrigerant contained therein during the heating cycle and having the usual active heat pump circuit including the first and second heat exchangers. The invention provides a heat pipe containing a second refrigerant and having a first normally higher end in the first passageway and a second, normally lower end in the second passageway, the second refrigerant being capable of being evaporated by said relatively warm air in the second passageway and being capable of being condensed by cold air in the said first passageway.