heat pump
A heating system mechanically transferring heat from nearby air or water into a building, with geothermal heating listed separately.
A heat pump is an engine which transfers heat from one location to another. When used as a heating system in a building, it must rely on the presence of a body of air or water outside the building from which the system can extract heat. Geothermal heat is often supplied by a heat pump, but is listed as a separate heating type.
The system moves heat from its source into the building. The outdoor source can be colder than the indoor space; the pump supplies the work needed for the transfer.
Heat drawn from the ground retains the separate geothermal-heating designation described above, even when a heat pump performs the transfer.