Ground-Source Heat Pumps: Tapping the Earth's Mass

Ground-source heat pumps (GSHPs), often called “geothermal heat pumps,” exploit the relatively stable temperatures found just 5 feet (1.5 m) or more below the surface, either depositing or extracting low-intensity heat. Heat pumps—whether ground-source or air-source—are basically air conditioners that can be run in reverse to provide heating as well as cooling.

Air conditioners (and refrigerators) make a relatively cool place cooler by extracting heat from it and releasing the heat in a warmer place. The key to this magic is the refrigerant, a fluid that changes from a liquid to a gas at ambient temperatures. After absorbing heat, the gas moves through a mechanical compressor that squeezes the heat back out, not unlike squeezing water out of a sponge. By releasing this heat to the temperate earth rather than into hot outdoor air, GSHPs cool more efficiently than air conditioners or air-source heat pumps. They provide heat by using the same principle in reverse—drawing heat from the relatively warm earth rather than from cold outdoor air.