Ground-source heat pumps have been around for decades but every year seem to attract more homeowners and organizations who are looking for alternatives to traditional space heating and cooling. They can hook into existing forced hot air and hot water systems but not steam heat.

1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.

A stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.

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or learn on Wikipedia about ground source heat pumps.