Ground source heat pumps rely on an energy exchange between the air within the building being heated and the ground. Ten feet below, the earth's temperature is fairly constant, generally around ~10ºC (~50ºF).

Ground source heat pumps rely on an energy exchange between the air within the building being heated and the ground

During the summer when the ambient temperature of the building exceeds that of the ground, heat pumps are used to pump heat from the building in to the transfer medium (typically water with small amounts of ethanol or glycol) and is subsequently pumped through narrow pipes into the ground so that the heat can be dissipated in the earth.

When the ambient temperature falls below the ground temperature the process works in reverse. Heat pumps extract heat from the ground and use it to heat the building.