Tempering valves mix some cold water with the domestic hot water leaving the tank, so that the water at the faucets is not too hot. These valves are designed to be installed close to the water heater, or at an acceptable distance from a plumbing fixture. Valves may be installed horizontally or vertically. Tempering valves blend hot and cold water to deliver mixed water at a constant temperature. They incorporate a temperature sensitive element which expands and contracts depending on the temperature of the water flowing across it. This action of the element in turn moves a piston which shuttles between the hot and cold ports. In doing so the valve maintains an essentially constant outlet temperature, and thus reduces the risk of scalding accidents.