A conventional storage water heater has a small valve screwed into its tank that is designed to relieve built-up steam pressure if the water heater overheats--othewise, the pressure could cause the water heater to explode.
This valve has several names, including temperature and pressure relief valve, pressure relief valve, T&P valve, or--in its shortest form--TP valve. As TP valves get old, they can begin to drip or leak and may need to be replaced.Though it is recommended that you periodically test this device by lifting its little lever to make sure it is working, doing this with an old valve can cause it to leak. With this in mind, be ready to shut off the water supply to the water heater and replace the TP valve if this happens when you test.
Note: Be aware that a pressure relief valve is designed so that water will pour out of it if pressure builds up too high in the water heater. With this in mind, the valve should be piped outdoors or to a drain.
Here is how to replace a TP valve:
1. Shut off the cold water and gas or electricity coming to the water heater.
2. Drain the water from the tank until the water level is below the valve. To do this, run hot water at any faucet, shower, or bathtub.
3. Raise the lever on the valve to release any excess pressure. Loosen the valve by turning it counter-clockwise with a wrench, and then remove it slowly. If water begins to pour out, screw it back in and drain more water from the tank. Note: The water may be hot unless you've allowed time for it to cool!
4. Put pipe-thread tape on the threads of the TP valve and then screw it in, turning clockwise. Tighten it with a wrench.
5. Turn the cold water supply and gas or electricity back on and, for a gas water heater, relight the pilot light if necessary.
