The pressure relief valve — formally Temperature and Pressure Relief Valve (T&P) — is the most important safety device on a water heater. It opens automatically if tank pressure exceeds 150 psi or temperature exceeds 210°F, dumping water through a discharge tube to prevent the tank from exploding. Required by code on every water heater. Test annually.
How it works
The T&P valve is a spring-loaded valve installed in a port on top or side of the tank. Two sensing mechanisms:
- Pressure sensor: a spring-loaded poppet opens when system pressure exceeds 150 psi
- Temperature sensor: a probe extends into the top 6" of the tank. If water reaches 210°F, the probe element expands and opens the valve
A discharge tube connects to the valve outlet and routes water to within 6" of the floor (most jurisdictions) — so an opened valve doesn't spray scalding water.
Why this matters
Without a T&P valve, an over-pressurized water heater becomes a steam bomb. If the tank reaches ~250°F under pressure and then a path opens (tank failure), all the water flashes to steam instantly — releasing enough energy to launch a 50-gallon tank through a roof. This isn't theoretical; the 1980 documentary footage of unrelieved water heater explosions is required viewing in plumbing trade schools.
The T&P valve is the single piece of plumbing equipment most likely to be quietly saving your life right now.
Annual test
- Stand to the side (not in front of) the discharge tube outlet — water that exits may be hot
- Have a bucket ready under the discharge tube outlet
- Lift the test handle on the valve for 2-3 seconds
- Water should discharge through the tube briefly, then stop when you release the handle
- Reseat the valve by pushing the handle fully down
| Result | Action |
|---|---|
| Water discharges normally, stops when handle reseats | Valve OK |
| No water discharges | Valve sediment-stuck or failed — replace |
| Water continues to drip after reseating | Sediment under the seat OR failed valve — try again, replace if persists |
| Heavy continuous flow that won't stop | Valve completely failed — shut off water supply, replace immediately |
Why is my T&P valve dripping?
If you find water dripping from the discharge tube, there are three common causes:
1. Sediment under the seat
Mineral particles lodge between the valve seat and poppet. Open the valve fully via the test handle 3-4 times to flush sediment. If it stops dripping, OK. If not, replace.
2. Excess pressure from thermal expansion
Water expands ~2% when heated from 50°F to 120°F. In a closed plumbing system (one with a backflow preventer, check valve, or PRV at the meter), this expansion has nowhere to go — pressure rises to 150+ psi and the T&P valve opens to relieve. The fix: install an expansion tank to absorb thermal expansion.
3. Failed T&P valve
Valve internals corroded; spring weakened. Replace.
How to replace
Tools: Pipe wrench or large adjustable wrench, replacement valve ($15-40), thread sealant (Teflon tape or pipe dope).
- Shut off cold-water supply to the heater
- Turn off power/gas
- Open a hot tap to relieve pressure
- Open the drain valve briefly to lower water level a few gallons below the T&P valve location
- Disconnect the discharge tube (loosen the union or unscrew the fitting)
- Using a pipe wrench, unscrew the old T&P valve counter-clockwise
- Clean the threads in the tank port
- Apply Teflon tape (3-4 wraps) to the new valve threads
- Screw in the new valve, hand-tight, then snug with the wrench. Don't overtighten — risk of cracking the tank port
- Reconnect the discharge tube
- Restore water supply; check for leaks at the new valve
- Restore power/gas
- Test the new valve via the lift handle
The discharge tube rules
Code requirements vary by jurisdiction but typically:
- Same diameter as the valve outlet (¾" typical)
- Routed downward only (no traps or upward bends)
- Terminates within 6" of the floor, or to a floor drain
- Cannot terminate inside a sealed enclosure
- Cannot be threaded at the end (so it can't be capped)
- Must terminate where discharge would be visible (so you notice if it's leaking)
T&P valve and warranty
The T&P valve is considered a wearable item and not typically covered under the parts warranty. Homeowner-replaceable; replacement is straightforward DIY.
Related guides
- Expansion tank — required if T&P drips from thermal expansion
- All parts
- Annual maintenance
- Water heater leaking
Bottom line
Test the T&P valve annually — 30 seconds. If it drips, suspect thermal expansion first (install an expansion tank), sediment second, valve failure third. Replacement is a $15-40 part and a 30-minute DIY. This is one of the few water heater parts where failure can be catastrophic; don't skip the annual test.