How does geothermal keep you comfortable all year? A geothermal system uses small-diameter, High-Density Polyethylene (HDPE) pipes buried underground called a "loop." The loop circulates water to move heat energy to and from your home. It's the heart of a geothermal system and the biggest advantage over ordinary heat pumps, air conditioners, and fossil fuel furnaces. | WaterFurnace Geothermal Heating, Cooling & Hot Water