Abstract: An air-to-air heat pump with improved efficiency of operation, and simplicity and economy of construction. Initiation and termination of defrost cycles during heating mode operation is controlled, in addition to the conventional clock timer, by a dual pressure switch responsive to pressure in the outdoor coil, thereby significantly reducing the length of defrost cycles with corresponding improvement in operating efficiency. The direction of refrigerant flow between the indoor and outdoor coils is established in a first line through the compressor by an electrically operated reversing valve, and alternatively through second and third lines, depending upon whether the system is operating in the heating or cooling mode, by series-connected check and float valves in each of such lines. When heating mode operation is resumed following a defrost cycle, the float valve has delivered all liquid-refrigerant to the indoor coil.

Abstract: This invention reduces the time length of defrost cycles in contemporary air-to-air heat pumps by a revision of the system refrigerant circuit wherein two parallel refrigerant circuits connect the reversing valve to the outdoor coil. One circuit includes a trap type accumulator with a check valve in its outlet conduit providing single direction refrigerant flow therethrough only during heating cycles, and the second circuit includes a check valve for single direction refrigerant flow from the reversing valve directly to the outdoor coil bypassing the cold accumulator during defrost cycles to prevent its interception of hot gas from the indoor coil required for quick defrosting of the outdoor coil.