A Temperature and Pressure Relief Valve is supplied with all water heaters. Its function is to provide both thermal and pressure relief when required. This will be in the form of discharging water from the valve. There are four types of flow which could be observed from the T&PR Valve.
Normal Operation When water is heated through a 50°C temperature rise, its volume will expand by approximately 2%. This expansion is discharged through the T&PR valve. It is normal operation for the T&PR valve to discharge or dribble water during the heating cycle. The T&PR valve will discharge water equivalent to 2% in volume of the total hot water used in one day. Therefore, a drain line must be connected to the T&PR valve to take discharge clear of the heater to a suitable termination point. This first type of discharge occurs only when a non-return or check valve is installed on the cold water supply line after the stop cock. It is normal and desirable that this valve allows a small quantity of water to escape during the heating cycle. However, if it discharges more than a bucket full of water in 24 hours, there may be another problem.
Continuous dribble If there is dirt or grit present under the seat of the valve, it may prevent the valve seat from closing properly. Therefore, a continuous dribble may be experienced. This dirt or grit may be dislodged by operating the easing lever and allowing a full flow of water to wash over the valve seat.
Steady flows for long period (often at night) It may be possible for the incoming cold water pressure to be in excess of the pressure rating of the T&PR Valve. This will result in a continuous flow of water from the T&PR Valve without cycling. Ask your Anode Swap technician to fit a pressure limiting valve. NEVER replace the relief valve with one of a higher pressure rating.
Heavy flows of hot water until water heater is cold – then stops until water reheats The T&PR Valve also operates as a temperature activated safety device. If by chance both the thermostat and over temperature energy cut-out fail on a water heater, then the electrical element or gas burner would operate continuously. Therefore, when the water temperature reaches 98-99°C, the T&PR Valve will open allowing a flow of water in quantity discharging the full capacity of the water heater then stopping, only to repeat this at intervals. This thermal relief is provided under high temperature conditions by expansion of the polythene rod inside the temperature probe, which causes the stainless steel push rod to lift the valve off its seat. The valve stays open until cooler water surrounds the temperature probe, allowing the polythene rod to contract. If this thermal relief does occur, the water heater must be switched off at the switchboard if it is an electric water heater. If it is a gas water heater, the gas control must be turned off using the knob on top of the gas control thermostat.