How It Works

Ground-source heat pumps differ from air source heat pumps only in that they exchange heat with the ground instead of outside air. In winter heating mode, heat is removed from the ground; in summer cooling mode heat is dumped into the ground.

Due to the immense thermal capacity of the earth, while the temperature of outdoor air ranges from over 100ºF down to -30ºF, the temperature of the earth at depths of 20 feet or more is the annual average air temperature for the location. Except for the most southern states, this temperature ranges between 45ºF and 60ºF.

Heat pump efficiency is strongly dependent on source temperature, so in the coldest months ground source heat pump efficiencies are much greater than those of air source heat pumps. In the northernmost states, heating season performance factors (HSPF), the amount of heat energy moved divided by the electrical energy consumed, varies from 250% to 350%. Excepting areas with very low gas costs or very high electricity costs, the ground source heat pump is the most economical HVAC system to operate.

High thermal efficiency comes at a cost, however. Installation costs are up to five times those of gas or oil systems. Most of the difference is due to the added cost of the underground piping (ground loop). The three most common loops are the vertical loops, the slinky loops, and the horizontal loops. The slinky is lowest in cost and lowest in efficiency. The horizontal loop is the most efficient where there is sufficient land available. Vertical loops are used where lot size prohibits the other two.

Adapted from How Your House Works, 2nd edition, available through RSMeans.