Abstract: This invention is an improved refrigerant-cycle heat pump that requires neither a defrosting means nor a backup heat source. From widely varying outside environments, it pumps heat to heat living spaces or domestic hot water. Liquid refrigerant flows from a receiver through expansion valves into an outside evaporator consisting of a segmented array of plates where heat is absorbed from the atmosphere to evaporate the refrigerant. The resultant gas is compressed by a motor-driven compressor, gives up heat in a condenser, becomes liquid, and returns to the receiver. The segmented evaporator enables the self-governing heat pump system to choose the proper magnitude of evaporator area, in response to its determination of compressor suction pressure or motor power demand, so as to provide a steady refrigerant flow to the compressor and thereby maintain optimum heat pumping despite extreme excursions of outside temperature.