How to Test Pressure Relief Valve
Updated February 20, 2026
Test your water heater's T&P relief valve in 2 minutes to make sure it will actually open when it matters -- plus how to replace a failed valve.
Overview
The T&P (temperature and pressure) relief valve is the most important safety device on your water heater. If temperature or pressure exceeds safe limits, this valve opens and dumps water through the discharge pipe before the tank becomes a bomb. A failed T&P valve on an overheating tank can cause a catastrophic rupture -- a steam explosion that can level a house. Most homeowners never test theirs, and valves sit for years corroding shut until they fail when it matters most. Testing takes 2 minutes and should happen once a year.
What You'll Need
Safety First
- The water coming out will be hot -- up to tank temperature (120-140 degrees). Stand to the side of the discharge pipe, not in front of it. Bucket at the end or make sure it drains to a floor drain. Gloves.
- Valve will not close after testing (water keeps flowing)? The seat may be fouled with mineral deposits. Do not force it. Turn off the heater and cold water supply, then replace the valve. A valve that will not close is safer than one that will not open, but it needs immediate replacement.
- Never cap, plug, or remove the T&P valve or discharge pipe. That removes the only thing preventing a catastrophic tank failure. Valve discharging during normal operation? The cause is excessive temperature or pressure -- diagnose the underlying issue, do not disable the valve.
Step-by-Step Instructions
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Locate the T&P Valve and Discharge Pipe
Side or top of the tank. Metal lever (usually brass) that you can lift, with a discharge pipe running from the valve down to within 6 inches of the floor. No discharge pipe? That is a code violation and a safety hazard -- install one before testing. The pipe prevents hot water from spraying on anyone during a test or actual relief event. Bucket at the end to catch the test water.
Tip: Cannot find it? Check both side and top. Power vent and high-efficiency units often put it on top. Look for a metal lever labeled 'T&P' or 'Temperature and Pressure Relief.' -
Lift the Test Lever to Open the Valve
Stand to the side, not in front of the discharge pipe. Lift the lever straight up about 90 degrees. You should hear water rush through and see it discharge into the bucket or drain. Strong, immediate flow confirms the valve opens properly. Let it flow about 5 seconds, then release. The lever should snap back and the flow should stop completely within a few seconds.
Tip: Lever stiff or hard to lift? Valve may be partially seized from mineral buildup. Gentle, steady pressure. Will not move at all? Do not force it with pliers -- the valve needs replacement. Forcing a corroded valve breaks the lever or damages the body. -
Evaluate the Test Results
Pass: lever lifts smoothly, water flows strongly while raised, flow stops completely when released. Fail: lever will not lift (seized), no water flows (blocked internally), water keeps flowing after release (seat fouled), or only a weak trickle (partially blocked). Any failure means immediate replacement. A non-functioning T&P valve is a serious safety hazard.
Tip: Small drip for 10-20 seconds after release is normal -- residual water in the pipe. Steady drip after 30 seconds means the valve is not resealing. Lift and release the lever 2-3 more times to flush debris off the seat. Still dripping? Replace it. -
Replace a Failed T&P Valve (If Needed)
Heater off (gas control to OFF or breaker off). Cold water supply off. Open a hot faucet to relieve pressure. Drain 2-3 gallons from the bottom valve -- just enough to drop below the T&P valve level, not the whole tank. Disconnect the discharge pipe. Pipe wrench to unscrew the old valve (counterclockwise). Teflon tape on the new valve threads (4-5 wraps), thread in and tighten (1-2 turns past hand-tight). Reconnect the discharge pipe. Cold water back on, refill, restore power. Test the new valve.
Tip: Match pressure and temperature ratings to the old valve (usually 150 PSI / 210 degrees for residential). Match the thread size (typically 3/4 inch NPT). Bring the old valve to the store if unsure. $15-30 for a replacement.
Pro Tips
- Test once a year. Mark the date on the heater with a marker. Many plumbers recommend testing when you change the clocks and swap smoke detector batteries -- easy to remember.
- Replace every 5 years proactively, even if it passes the annual test. $15-30 for a valve. The internal spring and seat degrade over time, and a valve that passes a manual test might still fail under actual overpressure if the spring has weakened.
- Valve discharging during normal operation? Do not assume it is defective. Most common causes: thermal expansion (water has nowhere to go in a closed system -- expansion tank fixes it), high street pressure (pressure reducing valve), or failed thermostat causing overheating. Diagnose the cause before replacing the valve.
- The discharge pipe must terminate within 6 inches of the floor. No caps, no plugs, no connection to the drainage system. Same diameter as the valve outlet (typically 3/4 inch). These rules ensure hot water discharges safely and visibly.
- No T&P valve at all? Extremely rare on modern heaters but possible on very old ones. Critical safety issue. Have a plumber install one immediately.
When to Call a Pro
Testing is simple DIY. Call a plumber if the valve is discharging during normal use and you cannot identify the cause, if you are not comfortable replacing it, if the tank opening is corroded, or if you have concerns about the overall condition of an aging heater.