High pressure may cause excessive discharge and possible premature failure of the operating relief valve (see table). The maximum water pressure usually occurs during the night, at the time of lowest water usage. In any mains pressure water heater installation if the water pressure exceeds 80% of the nominal set pressure of the operating relief valve, a Pressure Limiting Valve must be fitted to the cold inlet. If a cold water expansion control valve is fitted it will have a lower set pressure than the PTR valve and therefore will be the main operating relief valve.