[dougb]

They don't depend on geothermal activity. They depend on the relatively constant temp of the ground below the freeze line. In most places if you go down a few feet the earth is 40-50 degrees F. Its basically a heat pump that uses water instead of air.

My sister outside of Pittsburgh, PA has had a https://www.waterfurnace.com for 17 years. Her system has paid for itself a few times over. She has 1500 ft of plastic tubing burried in her front yard.