Most Likely Causes (Ranked)
The T&P (temperature and pressure) relief valve opens when tank pressure exceeds 150 PSI. This happens when there's no expansion tank in a closed system (homes with a check valve, backflow preventer, or pressure-reducing valve on the main). When the water heater heats water, it expands. In a closed system, that expansion has nowhere to go — pressure spikes and the T&P valve relieves it.
If the thermostat fails or is set too high (above 210°F), the T&P valve opens to prevent the tank from becoming a steam bomb. This is a genuine safety activation — the valve is doing exactly what it should. Gas water heater thermostats can fail in the 'on' position, causing continuous heating.
T&P valves have a spring-loaded mechanism that should reseal after each opening. After years of mineral buildup, intermittent activations, or simple age, the internal seat deteriorates and the valve weeps constantly — even when pressure and temperature are normal. The valve is a wear item with a 5-10 year lifespan.
This is the most common cause nationwide. When your water utility installs a backflow preventer or when a pressure-reducing valve is installed, the plumbing becomes a 'closed system.' Heated water expands but has nowhere to go. Pressure builds to 150+ PSI during every heating cycle, triggering the T&P. The solution is an expansion tank — not a new T&P valve.
Mineral sediment or pipe debris can lodge under the T&P valve seat, preventing it from sealing completely. Manually lifting the T&P lever (opening it fully, then releasing) sometimes flushes the debris and resolves the drip.
How to Fix It
Install an Expansion Tank
$40-60 DIY / $150-300 Pro- 1Check if you have a closed system: look for a check valve, backflow preventer, or PRV on the main line.
- 2If yes: install a thermal expansion tank (like Watts PLT-5) on the cold water line above the water heater.
- 3The tank has a pre-charge of air (match to your system pressure using a bicycle pump and pressure gauge).
- 4The tank absorbs thermal expansion, keeping pressure below 80 PSI during heating.
- 5This is the correct fix in 60%+ of T&P leak cases — not replacing the valve.
Replace the T&P Valve
$15-30 DIY / $150-250 Pro- 1If the valve is simply worn out (system pressure is verified normal): turn off power and close the cold water supply.
- 2Place a bucket under the discharge tube. Unscrew the old valve with a pipe wrench.
- 3Wrap new valve threads with 6 turns of Teflon tape. Install and tighten firmly.
- 4Reattach the discharge tube — it must run downward and terminate within 6 inches of the floor. Never cap or plug it.
- 5Turn on water, restore power, and monitor for 24 hours.
Flush Debris from the Valve
Free- 1Place a bucket under the discharge tube end.
- 2Lift the T&P lever fully and hold for 5-10 seconds to flush debris.
- 3Release. If the drip stops, debris was the cause.
- 4If it continues dripping, the valve seat is worn and needs replacement.
When to Call a Professional
NEVER cap, plug, or remove a T&P valve. It prevents catastrophic tank explosion — water heaters without functioning T&P valves have launched through roofs.
A T&P valve discharging boiling water is doing its job. Fix the underlying pressure or temperature issue — don't just replace the valve.
If the T&P is discharging forcefully with steam, the water heater is dangerously overheating. Turn off the gas/power immediately and call a plumber.
The discharge tube must not be capped, reduced in size, or terminated more than 6 inches above the floor. Code requires an air gap at the termination.
How to Prevent It
Test the T&P valve annually: lift the lever briefly and verify water flows, then release and verify it seals. This prevents mineral buildup on the seat.
Check your system pressure with a gauge ($10) annually. It should be 40-80 PSI.
Replace the T&P valve every 5 years as preventive maintenance ($15 and 15 minutes).
If you have a closed system, check the expansion tank's air pressure annually with a tire gauge.
Frequently Asked Questions
A slow drip from a worn valve: not immediately dangerous, but fix it — the valve won't work properly in an emergency. A forceful discharge with steam: potentially dangerous — shut off the water heater and call a plumber.
Check for a PRV, check valve, or backflow preventer on the main water line. If you have any of these, you have a closed system and need an expansion tank. Many municipalities now require backflow preventers.
Yes — it's intermediate DIY. Turn off power and water, drain a few gallons from the tank, unscrew the old valve, install the new one with Teflon tape. Budget 30 minutes.
If a brand new valve leaks, the issue isn't the valve — it's the system. Check for thermal expansion (need expansion tank) or high incoming water pressure (need PRV). Replacing the valve without fixing the underlying cause is pointless.