Combined cycle air/air heat pump

An air-to-air heat pump for a building comprising:

• a first passageway having an inlet and an outlet both communicating with air outside the building, and means for moving air through the first passageway from the inlet to the outlet, said first passageway having a first heat exchanger constituting an evaporator for a first refrigerant contained therein during a heating cycle;
• a second passageway having an inlet and an outlet both communicating with relatively warm air inside the building, and means for moving air from the inlet to the outlet, said second passageway having a second heat exchanger constituting a condenser for said first refrigerant contained therein during a heating cycle;
• means for pumping said first refrigerant in an active heat pump circuit which includes said first and second heat exchangers, and pressure reducing means for maintaining the pressure of refrigerant higher in the second heat exchanger than in the first heat exchanger during the heating cycle;
• a reversing valve for reversing the flow of said first refrigerant during a cooling cycle;
• a heat pipe containing a second refrigerant, said heat pipe having a first normally higher end in the first passageway and a second, normally lower end in the second passageway, said 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;
• said first end being positioned in the first passageway upstream of the first heat exchanger whereby cold air is pre-heated by condensation of said second refrigerant before contacting said first heat exchanger so that frosting on the first heat exchanger in cold air is inhibited;
• and
• said second end being positioned in the second passageway upstream of said second heat exchanger.