How to Replace a T&P Valve

Every water heater, gas or electric, should have a temperature and pressure relief valve, commonly called a T&P valve. Your water heater isn't safe without one. It's so important, in fact, that many states require factories to install T&P valves rather than leaving the decision to homeowners and installers.

The T&P valve is crucial because a water heater is a pressurized tank, and all such tanks need a way to release pressure if it becomes too great. As water is heated, it expands, increasing pressure. If something goes wrong and the tank overheats, the pressure could rupture the tank. To prevent this, the T&P valve monitors both water temperature (210 degrees maximum) and tank pressure (150 psi maximum) and opens if either goes too high.

The T&P valve is the only protection against both excessive pressure and high temperature. Both electric and gas water heaters have thermostats, and electric ones also have overload switches (the red button above the top thermostat). But if the city raised its water pressure without telling you and if your water heater was set at a high temperature, the tank could rupture. Without a working T&P valve, a pipe could blow off the heater (after its plastic or solder melts) or the tank could explode.

Safety: If ever you turn on the hot water and see steam coming out of the faucet, your T&P valve has failed. Leave the faucet open, walk out of the house immediately and call a plumber. You do not want to be in the house, or even close to it, if the tank explodes.

In theory, the T&P valve should be tested every couple of years by raising the lever to see if water sprays out. (If it doesn't, it's time to replace the valve.) But this test is controversial because as the water comes out, a flake of iron or other material may jam in the valve, causing it to drip after the lever is released. In other words, you could ruin a good valve by testing it.

Still, the valve does need to be tested. What's the solution? Be prepared: when you test the valve, make sure you have a new one in hand, Teflon tape on its threads, and a pipe wrench close by.

Safety:
Never test your T&P valve unless it's attached to an overflow pipe that extends to just above floor level. Otherwise you may get a face full of hot water. Before you test it, put a small bucket under the overflow pipe opening to catch the water that comes out.

T&P Valve Trouble SignsKeep in mind that a dripping valve could indicate real problems, so check to be sure the water isn't unusually hot or the pressure unusually high. If you test the valve, though, and it won't stop leaking after you test it, replace it.

If water keeps pouring out full-force after you test the valve and close the lever, replace the valve. You can stop the flow of water by quickly turning off the cold water going into the heater, and then turn the heater off.

Valve Replacement Rule of Thumb

This is only a mechanical test of the valve's ability to release pressure. It doesn't test the sensor that monitors the temperature and pressure there's no way to test that part of the valve.So how do you know if the T&P valve will work properly if the water overheats? You don't. But you can reasonably expect the thermostat to last at least for the duration of the heater's warranty. If you have a 5-year warranty, make it a practice to replace the T&P valve every 5 years; with a 10-year warranty, every 10 years.

2. Release water pressure from the tank by raising the lever on the T&P valve. Let the tank drain through the overflow pipe for several minutes or until water stops flowing. Close the T&P valve.

3. Remove the T&P valve. Unscrew the overflow pipe from the old T&P valve with a pipe wrench. Then unscrew the old valve from the heater by using a large pipe wrench and turning counterclockwise. If the valve is on top of the heater, be careful not to damage the electrical wires, vent or existing water pipes.

If the valve is side-mounted, water may come out of the hole when you pull out the old valve. You can drain the heater to below the level of the valve by putting a hose on the heater drain and running it to the outside. This may not work if scale has built up inside the heater it can easily plug the drain.

4. Install the new valve. Put joint compound (which plumbers call "pipe dope") or Teflon tape on the threads of the new valve. Push the threaded end of the valve into the hole and turn it clockwise to screw it in. If you do this quickly, you won't spill more than a half cup of water. The water isn't under pressure, but have a rag close by to plug the hole in case of an emergency. Be sure to orient the new T&P valve's outlet in the same direction as the old one's, so that the overflow pipe will be in the same location. Reinstall the overflow pipe.

5. Turn the water and heater back on. Turn the cold water supply to the heater back on, and run the hot water in a bathtub again for 3 to 5 minutes or until you're sure all air is out of the water heater. Turn off the tub's water faucet, turn the power or gas back on, and check for leaks. You're done.

Rex Cauldwell is a third-generation electrician and a master plumber. He has written widely on both subjects, most notably in Fine Homebuilding magazine and the Journal of Light Construction. His book Wiring a House was published by Taunton Press in 1996.

TOOLS AND MATERIALS
Pipe wrench
T&P valve
Bucket
Teflon tape or pipe joint compound
Large sponge
Rags
Garden hose