An automatic valve designed to discharge when a preset pressure and/or temperature condition is reached. 1. Pressure Relief Valve. An automatic valve which opens and closes a relief vent, depending on whether the pressure is above or below a predetermined value. 2.Temperature Relief Valve. a. Fusible Type. A valve which opens and keeps open a relief vent by the melting or softening of a fusible element at a predetermined temperature. b.Manual Reset Type. A valve which automatically opens a relief vent at a predetermined temperature and which must be manually returned to the closed position. c. Reseating or Self-Closing Type. An automatic valve which opens and closes a relief vent when the temperature reaches a predetermined value. d. Vacuum Relief Valve. An automatic valve which opens or closes a vent for relieving a vacuum, depending on whether the vacuum is above or below a predetermined value. Frequently used in a hot water supply system.