It goes by several names: relief valve, pop off valve, pressure release, or temperature and pressure relief valve. In all probability, it has a few other names that are better left unsaid. Near the top of your water heater is an odd-looking contraption.
It has a body that looks like a faucet.
Instead of a knob, it has a lever. Most of these valves have a piece of tubing or pipe, usually PVC, running from the valve to near the bottom of the water heater to prevent scalding someone if the valve releases suddenly. The device is designed to release if you move the lever or if the pressure inside the water heater tank exceeds the valve’s ability to withstand it. Its purpose is to prevent the water heater from exploding if the internal pressure goes out of control.
Most of the time, this valve is a forgotten part.
Since the relief valve is almost never needed, it frequently is not noticed unless problems happen. These valves can start to leak. When this happens, it is time to replace them. This valve can leak because it is old, because the water heater is getting too hot, or because excessive pressure is building up from some other cause. The vast majority simply get old and start to leak.
Once or twice per year, you should consider checking the valve.
It is easy to check the valve. Lift the lever slowly until water begins to flow out of the relief valve. Return the lever to its original position to stop the flow. As long as the valve opens and closes properly without leaking, it should be fine. The risk of testing the valve is that it may start to leak after the test and require replacement. Because of this, many people choose not to ever test it.
After you discover the need to replace the valve, it is time to get started.
It is best to have the new valve in hand before beginning the replacement process. However, sometimes it is necessary to remove the old valve so you can have it with you when you buy the new one. This is not a problem except that it adds a lot of time to the job.
Turn off the power to the water heater.
All electric water heaters need the power turned off. Most gas water heaters have an electric component for pollution control and the electric pilot. On a gas water heater, it is also important to turn the thermostat down enough that the unit will not try to come while you are replacing the relief valve.
Turn off the cold water line feeding into the water heater.
You should find a valve near the water heater in the cold water line that can be closed. If not, you may have to shut off all of the water coming into the house. Once the water is off, you should open the hot water faucet nearest to the water heater. This will relieve the pressure from the tank and prevent hot water from spraying you as you remove the valve. Leave this faucet open until the project is completed.
The location of the relief valve matters.
If the relief valve is on top of the water heater, you can proceed to remove it. If it is located near on the side of the unit near the top, you will need to drain a little water from the tank. This will prevent a big mess needing to be cleaned up later.
If there is room to get a bucket under the drain coming from the valve, you may get by with opening the relief valve and letting it drain until the water stops running.
The way that always works is to attach a garden hose to the drain at the bottom of the water heater and remove about 1/4 of the water in the tank. For example, a 40 gallon tank would need about 10 gallons removed. It is not a big deal if you drain the entire tank, except it will take longer to refill and reheat. When you have drained the water, close the drain at the bottom of the tank and remove the hose if you used these.
Use an open end or adjustable wrench to remove the old valve.
Most of the time, these unscrew with minimal trouble. Make sure that the threads and area around where the new valve will be inserted are clean and free of debris. Use Teflon tape or joint dope to coat the threads on the new valve. This will help prevent leaks after you have installed the relief valve. Carefully screw the new valve into the water heater and use the wrench to make sure it is tight.
Turn on the water to start refilling the tank.
Check the hot water faucet that you had opened earlier to release the pressure inside the water heater tank. Let it run until the water runs smoothly without spits or gaps in the flow. Turn it off when this is the case. Turn on the electric power to the water heater. If it is a gas unit, turn the thermostat up to the desired temperature. Check for leaks and tighten the relief valve gently to stop them.
Attach the drain tube to the new valve.
If you are using new PVC to make a new drain tube, be sure to apply adequate glue to get a good bond. You do not want this drain flying apart when it is being filled with pressurized hot water if the relief valve ever opens.