A water heater temperature and relief valve or TPR (or T&P) 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 shuts down. If, for any reason, the thermostat or control regulator quits working and fails to stop, the temperature and pressure 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 the tank, cause the rupture even explosion.
All the tank-type heaters, whether heated by gas, oil or electricity must have a TPR valve, so it can 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.
The 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 connections if the problem is on the threaded part, or to replace this part entirely.
Also, when replacing a T&P valve do the following:
There are two reasons why the temperature and pressure relief valve is dripping.
The TPR valve is dripping when it opens due to excessive temperature and pressure. In such a situation, water expands its volume, and if the system is closed, water cannot go anywhere, except through the TPR valve and out.
The second reason for the relief valve discharge is when it is broken (faulty). The valve might not seal properly, get stuck due to sediment buildup, and remain open.
The problems with the leaks can be as small as wasted water and energy, to severe such as property damages.
In order to prevent damages from the TPR leak, it is recommended to install a discharge pipe. One end of the pipe is connected to the TPR valve while the other end should terminate a few inches above the floor drain or a pan.
Some experts do not recommend termination of the discharge pipe into a home drain or outside, because it is important to see and monitor when the discharge occurs, so there is time to react and deal with the problem.
The best way to prevent the TPR valve from opening due to increased pressure and temperature is to install an expansion tank on the plumbing system. This applies to closed systems.
A TPR valve is both pressure and temperature sensitive, so when installed, it will provide needed protection against extreme pressure and temperature, including rupture and explosion.
If the water inside the tank reaches boiling point (212 F), the water turns into steam, increases its volume, and releasing a considerable amount of energy, making the operation unsafe for the water heaters and surrounding.
When the TPR valve is correctly installed and working as it should be, and if the pressure and temperature go over the limit, the valve will discharge overheated water. At the same time cold incoming water enters, reducing an inside pressure and temperature.
If the BTU capacity of the TPR is lower than the BTU of the heater, the valve won’t be able to do its job correctly, increasing the risk of tank rupture, even explosion.
To make sure that the size is right, check the capacity found on the plate of the valve.
Note: Gas water heaters use BTU for the gas input or power, while electric models use kW. So, to convert kW to BTU, use this formula: 1 watt=3.413 BTU/hr. If the heating elements have the power of 5500 watts, this will give you 5500*3.413=18771 BTU/hr.