A: Nicholas Carnot first proposed the basic principle of the geothermal heat pump in 1824. This theory was advanced 30 years later when Lord Kelvin proposed that refrigerating equipment could be used for heating. But, because of cheap, readily available fossil fuels, the geothermal heat pump remained a researcher’s curiosity until the mid-1930s when several manufacturers designed custom systems using the geothermal heat pump for comfort heating. Following the delays caused by World War II, commercial production of geothermal heat pumps began in 1952 and production and use has continued to grow since.