Leaking T&P valves, why they leak, and how to fix.
Temperature and pressure (T&P) relief valves are designed to be a safety measure. If the pressure in the system becomes too high, the valve will open. This happens when the system is closed. This means a pressure regulator, or some device, prevents water from flowing in two directions. In most instances, there will be an expansion tank installed. Because when all the water is drained from the tank, like during a shower, the tank is filled with cold water. As the water heats to 120° it expands. If all the outlets are closed, the water will build up pressure in the system as the water expands. The expansion tank is designed to absorb this pressure increase. If the expansion tank has failed, and they often do, the T&P valve will open up and release some water and thus reduce the pressure on the system. When this happens over and over, the valve can get blocked and remain slightly open, resulting in a continuous leak. Often people assume that the T&P valve is the problem when it is typically a failed expansion tank. You can check your expansion tank by tapping it. If it sounds solid and full all the way thru, it has likely failed and will possibly leak itself—more on that in a different post.