Temperature and Pressure (T&P) Relief Valve

The T&P (Temperature and Pressure) relief valve is the primary safety device on every water heater. It opens to relieve pressure if tank pressure exceeds 150 PSI OR tank temperature exceeds 210°F. Without a functional T&P valve, overheating scenarios can cause steam-flash explosion. Test semi-annually; never block or plug.

Why T&P matters

Water at 212°F at sea level becomes steam if pressure drops to atmospheric. A water heater storing water at 200°F has tremendous stored energy — if the tank fails, that energy releases as steam expansion (the tank can rocket through the ceiling). The T&P prevents this by relieving pressure long before the temperature gets there.

Standard specs

• ¾" NPT male thread
• Rated 150 PSI / 210°F
• Universal — Watts, Honeywell, Cash Acme all fit any water heater
• $15-25 part

Semi-annual test procedure

1. Position a bucket beneath the discharge tube
2. Lift the lever briefly (1-2 seconds)
3. Water should discharge through the tube under pressure
4. Release the lever
5. Discharge should stop completely
6. If valve drips after release, it\'s failed — replace

Watch for: gunk or mineral buildup in the discharge tube can cause "delayed" stopping. Inspect the tube periodically.

When to replace

• Drips after lever test
• Visible weeping or continuous discharge
• Age over 5-7 years (preventive)
• After plumbing freeze that may have damaged seat
• After any over-pressure event documented in service log

Replacement procedure

1. Power off at breaker; verify with voltage tester (electric)
2. Close cold water inlet shutoff
3. Open hot tap downstream to relieve pressure
4. Catch residual water with bucket beneath T&P location
5. Disconnect discharge tube
6. Unscrew T&P from tank port (large pipe wrench)
7. Apply Teflon tape (3-4 wraps) to new valve threads
8. Install new T&P, hand-tight plus 1-2 turns with wrench
9. Reconnect discharge tube
10. Open inlet; verify no leaks
11. Restore power; test operation

Discharge tube requirements (code)

• ¾" copper or CPVC pipe (matching valve outlet size)
• Continuous downward route — no traps or upward sections
• Terminate within 6" of floor or other safe surface
• No threaded cap on discharge end — must be open
• Terminate in same room as water heater (visible discharge required)
• No reducing fittings — full ¾" diameter throughout

What causes T&P discharge

1. Thermal expansion (most common)

Closed plumbing system without expansion tank. Install expansion tank.

2. Overpressure

House pressure regulator failed or set too high. Test static pressure (should be 50-80 PSI). Replace or adjust PRV.

3. Failed T&P valve

Seat eroded; relieves below 150 PSI. Replace.

4. Genuine overheating

Stuck thermostat causing tank to overheat. Diagnose and fix the heat side first.

Critical safety: never plug or block

If T&P drips frequently, fix the cause (expansion tank, pressure regulator, thermostat). NEVER:

• Cap the discharge tube
• Plug the T&P valve outlet
• Reroute discharge to a closed container
• Replace T&P with a higher-rated valve
• Lock the relief lever closed

The T&P is the last-line safety against steam-flash explosion. Disable it and you\'re risking catastrophe.

Bottom line

Test T&P semi-annually. Replace if drips after lever test. Universal $15-25 valve fits any water heater. Most "T&P discharge" issues are actually thermal expansion — install an expansion tank rather than blaming the T&P.