As the name implies, its purpose is to vent excessive pressure from a water heater to keep it from exploding. T&Ps have been code for all water heaters since the early 1960s. They should be tested yearly by pulling up on the handle. There should be a good flow of water through the valve. If there isn't, or if the valve won't stop dripping afterward, it should be replaced.

Dripping T&Ps (that you haven't just tested) can merely be defective, or they may be harbingers of other problems within a water heating system.

One other important thing: T&Ps have a long probe that extends from their base. The picture at left doesn't quite show it because it's through the piece of cardboard used as a background. Similarly, though, that probe should go into the tank or quite close to it, or it won't sense dangerous conditions and cause the valve to open in time.