Temperature and Pressure (T&P) Relief Valves installed on Hot Water Tanks should be checked once a year. The valves are normally found on the side or top of the tank with a pipe to within a few inches of the floor. A malfunctioning valve can allow a catastrophic explosion to occur. A hot water tank (HWT) with a malfunctioning thermostat can allow extreme pressures to build inside the HWT. The resulting explosion can be spectacular as well as devastating to your family, home & neighbourhood.
The following is a couple of excerpts from 411Plumb website, http://411plumb.com/how-to-test-your-tp-valve/.
“In your home there is a rocket. It weighs between 300 – 1200 Lbs. and can, without even a moment’s notice launch itself through the roof of your home, fly several hundred feet in the air, and land a block away. This rocket hides in basements, closets, garages, attics, and even under kitchen counters. However, there is a simple test you should do at least once a year that can assure the rocket will never launch. The rocket is your water heater and you need to test the T&P Valve at least once a year to make sure it is working properly.”
“The test is simple. On the T&P Valve there is a small handle that lifts a pin in the center of the valve. Before testing make sure you are in a safe position where you will not be struck by the hot water discharge, and that you know how to shut off the water supply to the water heater. You don’t have to turn it off but you need to know how to do it as the valve when tested may not reseat properly causing water to leak. If this occurs you will need to be able to shut off the water until the valve can be replaced. All you need to do for the test is lift the handle so the valve opens then release it so it closes. If the valve is frozen so it does not open or, does not close it should be replaced ASAP. Sometimes the valve may not set properly after testing. Try opening it and allowing it to slam shut several times. If this works the valve is okay. If it doesn’t it should be replaced. This test does not ruin a valve. If the valve fails the test it was already bad and was not offering the protection you need. Regular testing may actually increase the life of the valve. Test the valve a minimum of once a year.”
If you still don’t believe, watch the following video.