A water heater pressure relief valve is a necessary component of your water heater. It helps to ensure that excess pressure doesn't build up causing the tank to explode. It works by activating a spring when the pressure builds and releases pressure out of the tank. If your pressure valve is corroded or broken, it's best to replace it right away.
Step 1 - Cut the Power
Before you begin working on your water heater, you will want to make sure the power is shut off. The water heater will almost always have it's own circuit. Locate the circuit in your breaker box and turn the power to the water heater off. Some older water heaters will simply plug in to an outlet. This isn't common anymore, but if this is the case for yours simply pull the plug.
Step 2 - Drain the Tank
Before you drain the tank, allow the heater time to cool off. Not doing this can result in serious burns as you drain scalding water. Once it's cooled, you can use a hose to drain an indoor unit. If you have a drain in the floor where the water heater is located, you can allow it to slowly drain in to the floor drain. Otherwise, use a large bucket and drain it slowly, disposing of the water one bucket at a time. The entire tank doesn't need to be drained. Just drain enough so that the water is below the level of the relief valve.
Step 3 - Remove Tubes
You will find tubes attached to the pressure relief valve. In most cases the tubes will screw off, but some models the tubes will just pull out, or need to be cut. If you must cut through the tubes, be sure you don't cut in to the tank itself.
Step 4 - Remove Old Valve
Use your wrench to remove the old valve. It needs to be screwed out in a counterclockwise direction. If the valve is especially corroded you may need to apply lubricant such as WD-40 to help loosen the valve from the threads. Take care in this so that you don't end up breaking off the stem of the valve as you remove it.
Step 5 - Install New Valve
Use the Teflon tape on the threads of the new valve. This will help to form a more cohesive bond with the threads. Insert the new valve in to the opening and use the wrench to screw this down until it's secure. Use the tape again on the ends of the tubes if they're the kind that screw on and off. If not, use the plumbing putty on the tube. Allow the putty time to dry based on the directions.
Step 6 - Finishing Up
Go ahead and clean up your work area before you turn the tank back on. This way you won't be around a hot appliance while you clean up. Cut the power back on to the water heater, and turn the heater back on.