Temperature and pressure-relief (TPR) valves are safety devices installed on water heating appliances, such as boilers and domestic water supply heaters. TPRs are designed to automatically release water in the event that pressure or temperature in the water tank exceeds safe levels.
Temperature and pressure-relief (TPR) valves should be installed on all storage water heaters operating above atmospheric pressure. Tankless water heaters must have TPR valves installed. Water heaters without this protection can produce explosions. They have been responsible for many deaths. A pressure-relief valve relieves excessive pressure that may develop in a closed storage tank. A temperature-relief valve responds to excessive temperatures and discharges scalding water from the storage tank.
A TPR valve should be installed in the shell of a water heater tank. It should be located in the top 6 inches of the tank. Typically, water heater tanks have an opening in the tank shell installed by the manufacturer. The TPR valve is located at the top of the tank, which contains the hottest water in the tank.
The valve must be set to open at the maximum working pressure of the water heater, or 150 pounds per square inch or psi (1,035 kPa), whichever is less. An over-sized valve would not be able to prevent pressure from exceeding the maximum capacity, and a dangerous situation could result. The consequences could include an explosive tank rupture accompanied by an instantaneous release of enormous thermal energy, which is stored in super-heated water inside the tank. It could propel a water heater like a rocket through multiple stories, including the roof of a dwelling.
The maximum working pressure in the water heater is 150 psi. That information can be found on the water heater's data plate. And it should never be above the allowable working pressure stamped on the TPR valve. Once the pressure in the tank reaches the pressure rating, the hot water heater pressure relief valve opens to release the water.
Temperature-relief valves must be set to discharge at a temperature not higher than 210° F (99° C). The valve is designed to dissipate energy at a rate equal to or greater than the energy/heat input rate of the water heater. A relief valve opens in proportion to the temperature and pressure forced upon its closure disk. The higher the temperature or pressure, the greater the force, and the more the valve opens.
Combination temperature and pressure-relief (TPR) valves do two things: (1) they open and release water out of the tank if the temperature exceeds 210˚F (just below the boiling point), and (2) they will open if the pressure in the tank exceeds 150 psi (the maximum normal operating pressure for a water heater.) It is essential to avoid excessively high water temperatures and pressures at a water heater tank.
Relief valves must be third-party tested. The certification mark is the indicator that the valve has been tested.