 In heating, ventilation and air conditioning (HVAC) systems, the term "heat pump" usually refers to vapor-compression refrigeration devices optimized for high efficiency in both directions of thermal energy transfer.That is, heat pumps able to provide heating or cooling to the internal space as required.Heat pumps are more efficient for heating than resistance heaters because most of the energy they release comes from the ambient environment, and only a fraction from the externally-supplied energy required to run the device.