device for transferring heat from a substance or space at one temperature to another substance or space at a higher temperature. It consists of a compressor, a condenser, a throttle or expansion valve, an evaporator, and a working fluid (refrigerant), such as carbon dioxide, ammonia, or a halocarbon. The compressor delivers the vaporized refrigerant under high pressure and temperature to the condenser, located in the space to be heated. There, the cooler air condenses the refrigerant and becomes heated in the process. The liquid refrigerant then enters the throttle valve and, expanding, comes out as a liquid–vapour mixture at a lower temperature and pressure; it then enters the evaporator, where the liquid is evaporated by contact with a comparatively warmer space. The vapour then passes to the compressor, and the cycle is repeated.

A heat pump can serve as a reversible system for heating and cooling buildings. See heating.