Heat pumps move heat from a heat source (ground, water, or air) to provide heating and cooling. Air-source heat pumps extract heat from outside air for heating and reject heat outside for cooling. Ground-source heat pumps use geothermal energy from the earth. Water-source heat pumps use water bodies as heat source/sink. Heat pumps are more efficient than resistance heating and provide heating and cooling in one system. Coefficient of performance (COP) measures heating efficiency; values of 3 or higher mean 3 units of heat output for 1 unit of input energy. For cooling, energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER) measures performance. Installation of air-source heat pumps requires outdoor unit placement with adequate airflow. Outdoor units should not face prevailing winds which can cause icing and reduce performance. Refrigerant lines connect indoor and outdoor units and must be properly insulated and sloped to prevent condensation. Installation of ground-source heat pumps requires drilling boreholes and installing polyethylene tubing loops. Boreholes typically extend 150 to 300 feet deep. Heat transfer fluid circulates through loops improving thermal contact with the ground. The boreholes must be grouted to maintain thermal contact and prevent groundwater contamination. System controls modulate heat pump operation to maintain setpoint temperatures. Auxiliary heating may supplement heat pump output during extreme conditions. The electrical load of heat pumps requires adequate service capacity. Defrost cycles periodically reverse the heat pump cycle to melt frost on the outdoor coil; defrost mode temporarily stops heating. Properly sized equipment and controls optimize performance and efficiency.