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. Pressure relief valve is also required on tankless water heaters per ANSI and CSA standards.
When water in the tank heater is
is rising. Once the temperature is
reached, the unit, thanks to the thermostat and control valve,
automatically shuts down. If, for any reason thermostat or control
working properly and fails to stop the heating, temperature,
pressure and volume will reach the dangerous values.
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 heater working without this 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 the excessive increase of the temperature and pressure within the system. Once the relieving point of the water heater 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 fully open to discharge the 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: The excess hot water and pressure built by thermal expansion should be taken by the expansion tank not released through the TPR regulator.
If you notice that after the large amount of hot water is used, the pressure relief valve discharges water, probably you have to install an expansion tank. Water expansion is probably being limited by a backflow-preventer valve or pressure regulator, therefore causing the TPR to open.
TPR valve must comply with ANSI and ASME codes, to be certified by a nationally recognized testing lab, to be properly sized and unless it is provided with the heater, it has to be properly 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 removed from its designated opening, never be altered, restricted, or blocked. No obstruction to be placed between the relief valve and the heater.
It is important that the BTU capacity of the TPR exceeds the BTU input of the heater.
When installing TPR valve, position it downward and install tubing for the discharge of the excessive water. The discharge pipe should be properly sized to terminate an adequate drain and connected to a drain (6" above the floor drain or drain pan) and with no contacts with electric part.
The opening for temperature and pressure relief valve is either on a side or top of the heater tank. Recommendation is to install the brand new T&P valve only.
To prevent any problem with 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 water heater relief valve fails to completely reset, 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 moderate or large volumes.
Solution for these problems is to remove relief valve and reseal connection if the problem is on the threaded connection or to fully replace this part.
Apply the Teflon tape or approved pipe sealant on threads and install T&P relief valve in the marked opening. The valve must be installed so the temperature sensing element is immersed in the water within the top 6" of the tank.
Connect a drain pipe to the valve outlet and run to a safe place for water disposal. Keep the length as short as possible with the same size as the valve connection.
...if replacing T&P valve
Turn off the power and gas supply
Shut off water supply
Drain the tank so the level is below TPR valve
Unscrew the old TPR valve
Apply the Teflon tape or appropriate pipe joint compound on the thread
Put the discharge pipe back
Turn the unit on and refill the tank
Restart the heater
(All pictures are courtesy of Amazon.com)