The Reliance pressure and temperature relief valve has been designed to provide protection against over pressurisation, which can occur within an unvented hot water system if the input temperature controls fail. The valve is designed to be fitted in the upper portion of the cylinder as this is where the hottest system temperatures will be found. If the system temperature rises beyond 95°C, the wax mixture within the element of the temperature probe will expand, causing the valve to lift off its seat and relieve water to waste. If the water pressure increases (without temperature increase) the valve will stay closed until the set pressure of the relief mechanism is reached, at which point the valve will open to relieve the excess pressure build-up.