Compact pressure and temperature relief valves designed to provide protection against over pressurisation, which can occur within an unvented hot water system if the input temperature or pressure controls fail. The valves are designed to be fitted to the upper portion of a hot water cylinder as this is where the hottest temperatures in the system will be found. If the system rises above 95°C, the wax mixture within the element of the temperature probe will expand, this will lift the valve off its seat and release water to waste, therefore reducing the pressure within the cylinder. Also 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 again open and the excess pressure is relieved. Some models come complete with a PTFE coating on the MBSP inlet thread which can save valuable time on installation.