1894 J. A. Ewing iv. 118 Any heat-engine will serve as a heat-pump if it be forced to trace its indicator diagram backwards.
1948 E. F. Obert xiv. 520 The reversed heat-engine cycle is called a refrigerator (and, also a heat pump) when the evaporator is used for cooling purposes..; the same cycle is called a heat pump (but not a refrigerator) when the condenser is used for heating purposes.