Abstract: A heat pump system with three heat exchangers, two of which are connected through a reversible expander and the third of which is connected to both of the first mentioned exchangers through an expander and check valve arrangement for refrigerant to flow from the third to either of the first two heat exchangers but not vice versa. A flow control valve selectively connects the other side of either of the first two heat exchangers to the suction side of the compressor while selectively connecting the other side of any one of the heat exchangers to the high pressure side of the compressor to form a refrigeration loop including two of the heat exchangers. Refrigerant flow through the heat exchanger not being used is blocked.