How to Test the Water Heater Pressure Relief Valve
The temperature and pressure (T&P) relief valve is the most critical safety component on your water heater. If the thermostat fails, this valve is the last line of defense preventing the tank from becoming a pressurized bomb. Testing it takes less than a minute and should be done at least once a year.
Time Required
2-5 minutes
Difficulty
Easy
Safety
Hot water caution
Hot Water Safety
When you test the T&P valve, hot water (up to 120-150°F) will discharge from the pipe. Keep your hands and body away from the discharge pipe opening. Place a bucket to catch the water, and wear closed-toe shoes. Never remove or cap the discharge pipe — it must always be in place to direct hot water safely to the floor or drain.
What Is the T&P Relief Valve?
Before testing, understand what you're working with and why it matters.
- Location: The T&P valve is a brass valve mounted on the top or upper side of the water heater tank. It has a spring-loaded lever on top and a metal discharge pipe that runs down the side of the tank, ending within 6 inches of the floor or piped to a drain.
- How it works: The valve has an internal spring calibrated to open at 150 psi of pressure or 210°F of temperature — whichever comes first. If either threshold is exceeded, the valve pops open and releases water to reduce pressure and temperature.
- Why it's critical: Without a functioning T&P valve, a thermostat failure can cause water to superheat well above boiling. The resulting pressure can rupture the tank. In extreme cases, failed water heaters have launched through roofs and ceilings like a rocket. This is not exaggeration — it's documented.
- The discharge pipe matters: The pipe directs hot water downward to prevent scalding. It should never be capped, plugged, or reduced in diameter. It must terminate within 6 inches of the floor or be piped to a safe drain location.
How to Test the T&P Valve
Place a bucket under the discharge pipe
Position a bucket or container at the end of the discharge pipe to catch the water that will be released. If the pipe runs to a floor drain, you can skip the bucket but stand clear of the drain area.
Lift the lever for 2-3 seconds
Standing to the side of the discharge pipe (never in front of it), lift the metal lever on the T&P valve straight up. Hold it for 2-3 seconds. You should hear a rush of air and/or see hot water flowing out of the discharge pipe.
Release and verify it seals
Let go of the lever and it should snap back to the closed position. The flow of water should stop completely within a few seconds. Watch the discharge pipe for a full minute to confirm there's no continued dripping.
Interpret the results
Pass: Water flowed when lifted, stopped when released. The valve is working correctly.
Fail — no flow: If no water came out, the valve is stuck or blocked. Replace it immediately.
Fail — continuous drip: If water continues dripping after release, the valve seat is worn. Replace the valve.
When to Replace the T&P Valve
- Every 3-5 years preventively: Even if the valve passes testing, the internal spring weakens over time and mineral deposits accumulate. Proactive replacement is cheap insurance — the valve costs $10-$20.
- Immediately if it fails the test: A stuck valve that doesn't release water is a serious safety hazard. A leaking valve wastes water and energy. Both scenarios require prompt replacement.
- When it discharges on its own: If you find water periodically dripping from the discharge pipe without anyone testing it, the valve may be responding to excessive temperature or pressure. Check your heater settings and home water pressure first, then replace the valve if the problem persists.
- Match the pressure rating: When buying a replacement, match the PSI and BTU ratings stamped on the old valve. Most residential water heaters use a 150 PSI / 210°F rated valve.
Replacing the T&P Valve
Turn off the heater and water supply
For gas heaters, turn the gas valve to "Pilot" or "Off." For electric, flip the breaker. Close the cold water inlet valve at the top of the tank. Open a hot water faucet somewhere in the house to relieve pressure.
Drain some water from the tank
If the T&P valve is on the side of the tank, drain enough water so the level drops below the valve. Attach a garden hose to the drain valve at the bottom of the tank and open it, or simply drain a few gallons into a bucket.
Remove the old valve and install the new one
Unscrew the discharge pipe from the old valve. Use a pipe wrench to unscrew the old T&P valve from the tank. Wrap the threads of the new valve with Teflon tape (3-4 wraps clockwise), thread it in by hand, then tighten with a wrench. Reconnect the discharge pipe.
Pro Tips
- •Test annually — put it on your calendar: Pick a consistent date each year, like the day you change your clocks. The test takes under a minute and could prevent a catastrophic failure.
- •Never cap or plug the discharge pipe: Some homeowners cap the pipe because it drips occasionally. This is extremely dangerous — it defeats the purpose of the safety valve. If the valve is dripping, find out why and fix the root cause.
- •Check home water pressure: If the T&P valve is discharging frequently, your home water pressure may be too high. Use a pressure gauge ($10 at any hardware store) on a hose bib. Normal is 40-60 psi. Above 80 psi, you need a pressure reducing valve installed on your main water line.
- •An expansion tank helps: If you have a closed-loop water system (with a check valve or pressure regulator on the main), thermal expansion can cause the T&P valve to drip. Installing a thermal expansion tank ($40-$60) absorbs the pressure increase and stops the dripping.
Frequently Asked Questions
What does a T&P relief valve do on a water heater?
The T&P relief valve is the single most important safety device on your water heater. If the thermostat fails and water temperature or tank pressure rises to dangerous levels, the valve opens automatically to release water and prevent the tank from exploding. Without a functioning T&P valve, a malfunctioning water heater can build enough pressure to launch through a roof.
How often should I test or replace the T&P relief valve?
Test the T&P valve at least once per year by briefly lifting the lever. Replace the valve every 3-5 years as a precaution, even if it passes the test. The valve has an internal spring that weakens over time, and mineral buildup can cause it to stick. A new T&P valve costs $10-$20.
Why is my T&P valve dripping or leaking?
A dripping T&P valve usually means one of three things: the water heater temperature is set too high (above 120°F), the water pressure in your home is too high (above 80 psi), or the valve itself is worn out and needs replacement. First, check and lower the temperature. Then test your home water pressure with a gauge. If both are within range, replace the valve.