The most common type of space heating system used in homes in the U.S. is forced air. In most cases, the air is heated by a fossil fuel furnace or heat pump. Adapting an existing system to use geothermal heat or designing a system for new construction is a fairly straight forward process. The system consists of a finned coil, normally located in the supply air duct, a motorized valve to control the water flow in response to a signal from a thermostat, piping to deliver the water to and from the coil, and a few associated plumbing and electrical components