In short
A T&P valve is the temperature and pressure relief valve on a water heater. It opens by itself at 150 psi or 99 °C and dumps water through a discharge pipe so the tank cannot burst. A valve that drips is reporting a problem: thermal expansion, high house pressure or a worn valve. Never cap or plug it.
Look near the top of any tank water heater and you will find a brass valve with a small lever and a pipe running down the side of the tank toward the floor. That is the T&P valve, the final safeguard if a thermostat or gas control sticks on. Its probe sits in the hottest water at the top of the tank.
What a T&P valve does and the numbers stamped on it
The valve has two independent triggers. A spring holds it shut until tank pressure reaches 150 psi, and a temperature probe forces it open if the water reaches 99 °C (210 °F), just short of boiling. Both ratings, plus a BTU-per-hour relief capacity that must meet or exceed the heater's input, are stamped on the metal tag attached to the valve.
Water held under pressure above its boiling point flashes to steam the instant a tank splits, which is how an unprotected heater can fail violently. The relief valve exists so that pressure and temperature never get that far.
Why a T&P valve drips: the three causes
- Thermal expansion: water expands as it heats, and in a 'closed' system it has nowhere to go; homes with a backflow preventer or a pressure-reducing valve on the water service are closed systems, and the usual cure is covered in do I need an expansion tank on my water heater
- High house pressure: the higher the supply pressure, the less room there is for expansion before the valve lifts; plumbers treat about 80 psi as the ceiling for a house, and a pressure-reducing valve brings it down
- A worn valve: scale from hard water on the seat, or a tired spring, lets it weep at normal pressure, and the valve is replaced, not repaired
A pressure gauge on the tank's drain valve or a laundry tap tells the first two apart. Pressure that climbs only while the heater is firing points to expansion; pressure that is high all day points to the supply. A steady stream of very hot water is different: shut the heater off and call, because the valve is working against overheating.
What home inspectors flag on the discharge pipe
The discharge pipe matters as much as the valve, and home inspectors write up these defects:
- No discharge pipe at all, so scalding water would spray sideways at whoever is standing there
- A pipe that is capped, plugged or fitted with a shut-off
- A pipe smaller than the valve outlet, or reduced along its length
- A pipe that runs uphill or has a dip, where water sits and corrodes the valve shut
- A pipe that stops well above the floor instead of ending just above it or at a drain
The pipe should be full-size, rated for hot water, slope downward all the way and end in plain view. Never plug a dripping valve to stop the mess. A plugged relief valve leaves a pressure vessel with no way out.
When to replace a T&P valve and what it costs
Replace the valve when it weeps at normal pressure, when it will not reseat after a test, when the lever is seized, or whenever the tank is replaced. Manufacturers ask for the lever to be lifted briefly once a year. A valve that has gone a decade untested may be seized or may not close again, so have a technician do the first test with a new valve on hand.
A relief valve swap is a minor repair inside the $150–$800 band that covers most water heater repairs, and an expansion tank or pressure-reducing valve is quoted separately if the gauge shows you need one. If the water is coming from the tank body and not the pipe, read why is my water heater leaking. ZK Mechanical handles both under water heater service: browse the HVAC glossary or call us with what you are seeing.
Questions people ask about T&P valve
Is a dripping T&P valve an emergency?
A slow drip is a reason to book service this week, not to shut the house down. Put a bucket under the pipe and note whether it drips only after long heating cycles. A valve releasing a steady flow, steam or very hot water is urgent: turn the gas control to off or switch off the breaker, close the cold supply and call. In neither case should the pipe be capped.
How do I test a T&P valve?
Lift the lever for a second or two with a bucket under the discharge pipe; water should rush out and stop cleanly when the lever snaps back. Stand clear, because the water is scalding. If nothing comes out, the valve is seized. If it keeps dribbling afterwards, grit is on the seat or the valve is worn. Either result means replacement.
Do tankless water heaters and boilers have a T&P valve?
A tankless heater has a pressure relief valve on its hot outlet, normally part of the isolation valve kit, because there is no stored volume to overheat. A boiler uses its own relief valve set at a much lower pressure, commonly 30 psi on a residential hot water boiler. The combined temperature-and-pressure type belongs to storage tanks, indirect tanks included.