Let’s relax a minute and figure out whether the T&P valve is doing its job or, if it needs replacement. There are two ways it could leak and be doing its job. The name of T&P Valve has meaning. “T” is for temperature and “P” is for pressure. The valve is the final safety device that can prevent a water heater from exploding.
T&P Valve Leaks Caused By Temperature
If the water temperature exceeds the boiling temperature of water the water heater could have a steam explosion. So the valve is set to open at 210 Degrees F and if the water temperature in the water heater were to cause T&P Valve to open there would be a fairly large release of water until the water in the water heater became cooled enough by the incoming water to allow the T&P Valve to reset. This would be an instance where the T&P Valve was doing its job and replacement is not warranted. Instead you should look at a failed thermostat that is not turning off at the set temperature, the upper thermostat should also have had the red reset button pop out as the reset button is a temperature activated device. Another problem could be an element that is shorted to ground. The 240 Volt supply is only switched on one side, so 120 Volts from the unswitched side could continue to flow even with the thermostat shut off through at least part of the element to ground and cause heating not controlled by the thermostat. If you were to replace the T&P Valve in this circumstance the new one would do the same thing the problem needs to be diagnosed and repaired.
T&P Valve Leaks Caused By Pressure
Water releases caused by pressure can be a little harder to diagnose and cure. They may be caused by the supply line pressure being too high or, thermal expansion within a closed system. In any case a pressure gauge with a “lazy hand” that screws onto a hose bibb will be useful in diagnosing pressure problems. The “lazy hand” is a hand which is pushed to the highest pressure recorded by the gauge needle and does not lower until you reset it. The first thing to check is the system pressure. By most codes if the pressure goes above 80 PSI. a pressure reducing valve and thermal expansion tank should be on the system.
To test the system pressure make sure the water heater is not heating the water in the tank. Run a small amount of cold water to release the build up of pressure due to thermal expansion or, a failed pressure reducing valve, screw the valve on a hose bibb and turn on the hose bibb. The reading you get is the system pressure. If it is greater than 80 PSI. a pressure reducing valve and thermal expansion tank should be installed. If there is already a pressure reducing valve installed it has failed and requires replacement. If it starts out less than 80 PSI. and slowly creeps higher then you probably have a pressure reducing valve that is leaking and allowing the pressure to increase past its set point and it should be replaced. If the supplied pressure approaches 150 PSI. the T&P will open and flow water as long as the pressure it too high. If you have at this point checked the temperature and pressure and both are within the normal parameters of the T&P Valve yet the valve is still open and flowing at this time replace the T&P Valve. Once this test is resolved you should proceed.
T&P Valve Leaks Caused By Thermal Expansion
Next we have to test for thermal expansion. Run a lot of hot water. Have someone take shower or take a bath. Watch the gauge as the water in the water heater is heated without running any water in the house and see what happens. If the pressure creeps up while the water is heating it is a sign that the water supply is a closed system and thermal expansion is causing the pressure rise. Generally the discharge of water from thermal expansion is a small amount like a coffee cup or, two. Any rise or, discharge from thermal expansion would indicate a thermal expansion tank should be installed. The thing you are looking at is the water heater is a pressure vessel. Cycles of pressure is taken into consideration of the design but it’s a lot like the nine lives of a cat. You use them and you loose them, there are just so many pressure cycles built into the tank before it fails and a thermal expansion tank absorbs the pressure instead of the water heater. Once this test is resolved you can proceed or, proceed with out using any hot water after the water heater has finished reheating and you have run cold water to eliminate any stored thermal expansion.
The next step is to watch the supplied pressure over a period of time. Leave the gauge on at least overnight without running hot water or, if you have resolved that thermal expansion is not a factor you can use hot water. Frequently at night water companies run higher pressures as they fill storage tanks to be ready to meet the water supply demands the next day. This is where the lazy hand is required so it can record any pressure spikes over an extended period of time. Again, if it goes over 80 PSI. a pressure reducing valve and thermal expansion tank should be installed. Ideally you would run this test for a 24 hour period.
T&P Valve Leaks Caused By T&P Valve Failure
If you have run through these diagnostic tests and determined that the leaking T&P Valve is not leaking for one of the above reasons replace it. Just a side note the actual numbers at which the valve is rated to open at has a fairly wide tolerance so it may open slightly before the rated temperature or, pressure. If the pressure is over 80 PSI. or, over the set temperature there is a problem that requires resolution.