Short answer
A T&P valve opens when the tank gets too hot or the pressure too high. Typical home valves, such as Watts’ 10L series, are rated 150 psi and 210 F. If it drips, the cause is usually thermal expansion on a closed system, high house pressure, too high a temperature, or a worn valve. The discharge pipe must end no more than 6 inches above the floor, a floor drain or a waste receptor, with no threads, valves or tees at the end (ICC). Never cap or plug it.
What it does
A T&P valve has a probe that sits in the hottest water near the top of the tank. It opens if water temperature reaches its set point or pressure reaches its rating. Watts’ 10L valve, a common example, lists 210 F temperature relief and a 150 psi pressure setting (Watts). Model numbers matter: some valves are set differently, so replace like with like.
Discharge pipe rules
The ICC’s CodeNotes on water heater safety summarizes the International Plumbing and Residential Code rules for the pipe from the valve (ICC). Among them:
- Discharge to the floor, to the pan serving the water heater, to a waste receptor or to the outdoors.
- Discharge where it cannot injure anyone or damage the building, and where occupants can see it.
- No traps or sags: it must drain by gravity.
- End no more than 6 inches above the floor, a waste receptor or floor drain.
- No threads at the end, and no valves or tees anywhere in the pipe.
- Pipe rated at least 100 psi at 180 F, from the listed materials.
When it drips: what to check
Dripping T&P valve: causes and fixes
| Cause | How to tell | Fix |
|---|---|---|
| Thermal expansion | Drips after the heater runs, on a system with a PRV, check valve or backflow preventer | Add or recharge an expansion tank |
| High house pressure | A gauge on a hose bib reads well above normal | Have the pressure reducing valve checked or adjusted |
| Temperature too high | Water at the tap is far above 120 F | Lower the setting |
| Worn valve or debris | Drips constantly at normal pressure and temperature | Replace the valve |
Testing the valve
Put a bucket under the pipe
The water that comes out is scalding. Keep hands and feet clear of the pipe end.
Lift the test lever
Raise it for a few seconds. Hot water should flow freely from the discharge pipe.
Let it snap back
Release the lever. The flow should stop within a moment.
Watch for a drip
A valve that will not open, or keeps dripping after testing, needs replacing. Old valves that have never been tested sometimes start dripping when first opened.
Replacing a T&P valve
Turn off gas or power, shut the cold supply, open a hot tap, and drain a few gallons from the drain valve so the water level is below the valve. Unscrew the discharge pipe and the old valve, wrap the threads of the matching replacement, screw it in with the outlet facing down, reconnect the pipe, refill the tank with a hot tap open until water runs steadily, then restore heat.
See 3/4 inch T&P relief valves
Questions people ask
Why is my water heater relief valve leaking?
The common causes are thermal expansion in a closed system (no working expansion tank), house water pressure that is too high, a temperature set too high, or a valve with debris on its seat or simply worn out. A.O. Smith also says a dripping T&P valve means it failed or the temperature is set too high.
How do I test a T&P valve?
Put a bucket under the discharge pipe, then lift the test lever for a few seconds so hot water flows, and let it snap back. If it does not flow, or keeps dripping after it closes, replace the valve. Follow your heater’s manual for how often to test.
Can I plug or cap a leaking relief valve?
No, never. The ICC notes the discharge pipe end must not be threaded, precisely so no one caps it. A blocked relief valve can let a tank fail violently.
What T&P valve do I need?
Match the old valve’s pressure and temperature ratings, thread size (usually 3/4 inch) and probe length, and the heater’s BTU or kW rating printed on the valve. The water heater manual lists the required rating.
Sources
Last reviewed October 29, 2026. Models, codes and listings change; check your heater’s manual and your local code, and follow the maker’s instructions.