Geothermal
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Heat pump: 
A mechanical device used for heating and cooling that operates by pumping heat from a cooler to a warmer location. Heat pumps can extract heat from air, water, or the earth. They are classified as either air-source or ground-source (geothermal) units.

Air source heat pump:
 A heat pump that uses air to extract or expel heat.

Geothermal heat pump:
 A heat pump that uses the earth as a heat source and heat sink that uses a loop of buried plastic pipe as a heat exchanger.

Heat sink: 
The medium – air, water or earth – which receives heat rejected from a heat pump.

Heat source:
 The medium (air, water or earth) from which heat is extracted by a heat pump.

Open-loop heat pump:
 A heat pump system that uses groundwater from a well. The water is then returned to the environment.

Payback:
 A method of calculating how long it will take to recover the difference in costs of two different heating and cooling systems by using the energy and maintenance cost savings from the more efficient system.

Desuperheater:
 A partial heat recovery system that captures heat from the refrigerant system as it leaves the compressor and transfers it to the domestic hot water. The process only provides hot water while the heat pump is operating.