Pressure-balancing valve
Also known as a pressure-compensating valve, this device is designed to reduce the risk of thermal shock and scalding while showering. Required by code in most areas of the United States, a pressure-balance valve senses the hot and cold water pressures coming in from the supply line and compensates for variations to maintain the water temperature. Such variations can occur when a toilet is flushed or a washing machine started while someone is showering.