A water heater’s temperature and pressure (T&P) relief valve drips for one of three reasons: the water pressure in the house is too high, thermal expansion has nowhere to go because the system is closed, or the valve itself is worn or dirty. It can also mean the water is too hot. Never plug the valve or its discharge pipe — it is the tank’s last line of defense.
Safety first
A valve that is discharging steam or a steady stream of very hot water means the tank may be overheating or over-pressurized. Turn off the power (electric) or set the gas control to off, shut the cold-water inlet, keep clear of the discharge pipe, and call a plumber. Do not cap, plug or tighten the valve to stop a drip. The discharge pipe should run downward and end a few inches above the floor, not into a bucket that could fill and hide a problem.
What the valve does
The T&P valve opens automatically if the tank gets too hot or the pressure climbs too high, releasing water through the discharge pipe. A dripping valve is therefore a warning signal, not just a leak.
The usual causes
| Pattern | Likely cause |
|---|---|
| Drips when the heater is heating, stops when idle | Thermal expansion in a closed system |
| Drips constantly, pressure gauge reads high | Household pressure too high (above about 80 psi) |
| Drips with very hot water, thermostat set high | Water temperature too high |
| Drips slowly at all times, with mineral crust on the valve | Worn seat or sediment on the valve |
| Water around the base, not at the valve | A tank or fitting leak — see water heater leaking: where it comes from |
Test the household water pressure
- Buy a hose-bib pressure gauge
It screws onto an outdoor tap or the washing machine tap and is inexpensive.
- Close all taps and read the gauge
Normal pressure is roughly 40 to 60 psi. Readings consistently above 80 psi stress the valve and every fixture in the house.
- Fix the pressure if it is high
High pressure often means a failed pressure-reducing valve at the water main — a plumbing job. See whole-house water pressure problems for the other direction.
Thermal expansion
When water heats it expands. If a check valve or pressure-reducing valve keeps the system closed, the expansion pressure has nowhere to go, so the T&P valve opens each heating cycle. An expansion tank on the cold supply absorbs it. If you already have one, a waterlogged or failed tank is the usual culprit; see expansion tank problems. Install or replace the tank with a plumber.
Check the temperature setting
A thermostat set very high or a failed thermostat can overheat the tank and open the valve. Most guidance puts the setting at around 120°F (49°C) for household use. Do not set it higher to compensate for hot-water problems; if water is scalding, turn it down and see water heater troubleshooting.
A worn or dirty valve
If pressure and temperature are normal and the valve still drips, its seat may be worn or coated with mineral deposits. Some people lift the test lever briefly to flush debris, but an old valve can fail to reseat afterwards and start leaking, so be ready to replace it. Draining the tank at the same time is a good moment to flush it (how to flush a water heater). Replacement valves are inexpensive; fitting one means working with a pressurized tank, so use a plumber if you are not sure.
When to call a plumber
Call a plumber for high household pressure, a missing or failed expansion tank, a valve that keeps dripping after replacement, or any steam or hot discharge. An old, leaking tank may not be worth repairing — compare with repair cost and signs you need a new water heater.
