Water Heater Temperature and Pressure-Relief (TPR) Valves
The thermostat in a water heater shuts off the heating source when the set temperature is reached. If the thermostat fails, the water heater could have a continuous rise in temperature and pressure (from expansion of the water). The temperature and pressure could continue to rise until the pressure exceeds the pressure capacity of the tank (300 psi). If this should happen, the super-heated water would boil and expand with explosive force, and the tank would burst. The super-heated water turns to steam and turns the water heater into an unguided missile.
To prevent these catastrophic failures, water heaters are required to be protected for both excess temperature and pressure. Usually, the means of protection is a combination temperature- and pressure-relief valve (variously abbreviated as T&P, TPV, TPR, etc.). Most of these devices are set to operate at a water temperature above 200° F and/or a pressure above 150 psi.
Do not attempt to test the TPR valve yourself! Most water heating systems should be serviced once a year as a part of an annual preventive maintenance inspection by a professional heating and cooling contractor.
