Abstract: The invention provides a heat pump system employing a refrigerant charged space and including at least two evaporators, one of which is an ambient air head absorber, and the other of which can be of the same type, or of the type obtaining heat from solar insolation, water, ground heat, etc. The coil or coils of an ambient air absorber must be defrosted periodically. In the system of the invention defrost heat is applied to the evaporator coil or coils while the compressor is operating, such defrost heat causing vaporization of the refrigerant therein. The system includes a pressure containment valve between the evaporator and the compressor that maintains the vapor pressure of the vaporizing liquid in the coil above the value at which the ice will readily melt, the valve releasing excess vapor to the compressor and then to the condenser which discharges the heat to the space to be heated. Thus, the only part of the defrost heat lost from the system is that required to melt the ice.