The Home Depot, Ace Hardware and True Value Hardware sell pressure-relief valves. Most pressure-relief valves also open if the tank temperature reaches 210 degrees Fahrenheit. They are available in various stem lengths to match the thickness of insulation between the outer tank cover and inner tank.
Pressure relief valves protect water heaters from explosions if the thermostat fails to stop heating the water. The out of control appliance eventually converts the water to steam and builds pressure beyond the tank's ability to contain it. As the pressure and temperature build, the relief valve opens to release the buildup. The plumbing of most tanks directs this excess hot water outside or to a floor drain; however, the owner should ensure the underlying problem is fixed.
Since pressure-relief valves are normally closed, they sometimes develop problems opening, reducing their ability to protect the tank. Periodic testing ensures the valve continues to operate properly. For safety, the tester ensures the gas or electricity is off and closes the water supply valve. After the unit has a chance to cool, the tester places a bucket under the end and lifts the lever. A properly operating valve makes the noise of a slight rush of air, or it releases steam or water. Valves that do none of these things require replacement.