A water heater relief valve or TPR valve is a safety device that is temperature and pressure sensitive and designed to limit its levels in the tank-type heaters. A pressure relief valve is also required on tankless water heaters per ANSI and CSA standards.
When water in the tank heater is heated, the temperature is rising.
Once the temperature is reached, the unit, thanks to the thermostat and control valve, automatically shut down. If, for any reason, the thermostat or control regulator quits working and fails to stop the heating, temperature, pressure, and volume will reach the dangerous levels.
The dangerous level is when the water is heated beyond its boiling point of 212 F. The overheated water is turning into steam instantly, increases its volume and releases the amount of energy enough to become a steam-powered bomb. This is why the heater, working without the safety elements can weaken a tank, cause the rupture and finally cause the explosion.
All the tank-type heaters, whether heated on gas, oil or electricity must have a TPR valve, to prevent an excessive increase of the temperature and pressure within the system. Once the relieving point of the relief valve is reached, it will release the excess pressure with the hot water and allow cooler water to enter the tank and lower the temperature.
The T&P regulator has a probe immersed in the first six inches of the water heater, to measure the stored water temperature. The temperature rating is set to 210 F. Once the probe senses an excessive temperature (i.e., due to thermostat failure) the relief valve will open fully and discharge hot water until the temperature is below its reset temperature.
Maximum working pressure in the water heater is up to 150 psi (it can be found on the water heater's date 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 will open to release the water. In plumbing and HVAC world this is also known as dribbling or weeping.
Note: An excess hot water and pressure built by the thermal expansion should be taken by the expansion tank not released through the TPR regulator.
If you notice that after a large amount of hot water is used, the pressure relief valve discharges water, probably you have to install an expansion tank. A backflow-preventer valve or pressure regulator are probably limiting water expansion, therefore causing the TPR to open.
A TPR valve must comply with ANSI and ASME codes, to be certified by a nationally recognized testing lab, to be correctly sized and unless it is provided with the heater, it has to be adequately installed.
Due to an overheating and extreme tank pressure that can cause serious injury, it is mandatory to install T&P safety regulator and must not remove from its designated opening, never be altered, restricted, or blocked. No obstruction to be placed between the relief valve and a heater.
It is important to have the BTU capacity of the TPR exceed the BTU input of the heater.
When installing the TPR valve, position it downward and install the tubing to discharge the excessive water. The discharge pipe should be appropriately sized and terminate to an adequate drain (6" above the floor drain or drain pan) and with no contacts with the electric part.
The opening for temperature and pressure relief valve is either on a side or top of the heater tank. The recommendation is to install the brand new T&P valve only.
To prevent any problem with a water heater relief valve, it must be manually tested at least once a year.
Note: When testing the relief valve, make sure no one is in front or around the outlet of the TPR valve discharge line as the water from the tank might be very hot.
If a water heater relief valve fails to to reset completely after lifting the lever, and continues to release hot water, turn the unit and water off and replace the TPR.
If you see a puddle of water around the base of the unit, leaking water heater relief valve might be one of the causes. Usually, water is seeping around the valve-tank connection, leaking at the threaded portion of the TPR valve connection and directly from the valve in the moderate or large volumes.
A solution for these problems is to remove the relief valve and reseal connection if the problem is on the threaded connection or to replace this part entirely.
Also, when replacing a T&P valve do next:
Drain valve - How to replace
Heating elements - Buying guide, testing and replacing
Thermostats - how to set, problems, testing and replacing
How to replace an anode rod
How to replace a thermocouple
Replacing a TPR valve
Dip tube - how does it work and replacing tips