RMC's Pressure and Temperature & Vacuum Relief (P & T Relief and P,T & V Relief) Valves are safety controls fitted to pressurised hot water systems. These valves ensure that the temperature of the water in a pressurised unvented water heater cannot exceed 99°C in the event that the normal heater thermostat controls fail. These valves guard against both over-temperature and over-pressure hazards wherever water is stored in unvented containers.