How to Test and Replace the TPR Valve on Your Water Heater

The temperature pressure relief valve on a water heater, also known as the TPR valve, is a safety device designed to prevent an excessive buildup of pressure and a possible explosion. If you're wondering whether your water heater has one, wonder no more — if it has a tank (as opposed to being a tankless, on-demand model), it has one.

The TPR valve is a brass fitting mounted on the top or side of the tank. It looks like a spigot with a 3/4-inch male threaded inlet and a 3/4-inch male or female threaded outlet. Instead of a handle, it has a spring-loaded lever, and when installed according to code, it has a discharge pipe that extends to a few inches above the floor.

Since the job of the water heater pressure relief valve is to release excessive pressure, most of the time, it isn't doing anything at all. It's made of brass, so it doesn't corrode, but scale can build up on the lever mechanism. When there's enough buildup to prevent the lever from closing all the way, the TPR valve can leak even when the tank pressure is in a safe range, and that's one reason to replace it. Another more important reason is when it won't open when it should.

It's one thing when the TPR valve leaks and drips a steady stream of water onto the floor, making a mess and filling the water heater room with humidity. It's another when the valve gets stuck in the closed position by scale and hard water deposits and doesn't do the job it's supposed to do, which is to open and relieve excess pressure. That can be catastrophic.

Water in the hot water heater tank expands as it gradually heats up, and if it doesn't have anywhere to go, it puts pressure on the hot water tank liner, which will eventually leak. In a worst-case scenario in which the water heater temperature rises past the safe limit of 120- to 140-degrees Fahrenheit because of a thermostat malfunction, the tank can literally explode, causing flooding and possible injury.

Expanding water backs up into the cold water supply pipe that extends into the tank, and an open path back to the water source greatly reduces the possibility of explosion. The main water valve on some homes, however, has a check valve that prevents backflow, and that creates a closed system in which pressure can definitely build to dangerous levels. Not only is a working TPR valve essential in this type of system but an expansion tank should also be installed on the cold water supply line.

Homeowners should test the water heater TPR valve every six months. The procedure is simple and involves opening the valve manually. Conduct the test on a day when you're sure hardware stores are open so you know you can get a replacement if something goes wrong.

Before testing the valve, make sure a discharge tube made of a corrosion-resistant material, such as copper, PVC, or CPVC, is connected and forms a drain line that extends down toward the floor. If there isn't a pan or bucket on the floor already (there should be), put one under the discharge tube opening to catch water. Lift the lever on the valve partway — just enough to allow water and steam to escape. Don't open the lever all the way or it may get stuck in the open position.

You should hear water bubbling into the discharge tube and see it pouring from the end of the tube. If no water comes from the valve, it's blocked and needs to be replaced immediately. If water continues to come out of the valve after you release the lever, lift the lever again and let it fall a few more times. If necessary, give the valve a few light taps. Replace it if you can't get it to close.

The power to the water heater must be off when you replace the TPR valve. You don't necessarily have to wait for the water in the tank to get completely cold, but you should definitely give it time to cool down somewhat to avoid getting scalded.