Abstract: A dual source heat pump system is disclosed for automatically selecting in either the heating or cooling mode of operation that source which will most efficiently meet the heating or cooling demand required of the system. The heat pump system includes valves located so that the heat exchanger associated with the source not selected can be isolated from the remainder of the system and temporary pumping cycles initiated at the conclusion of a heating or cooling cycle for pumping all the refrigerant into the indoor space exchanger coil. Virtually all the refrigerant charge is thereby maintained in the active portions of the heat pump system while the inactive portion is completely isolated.