Short definition
A T&P relief valve (temperature and pressure relief valve) is the required safety valve threaded into the top or upper side of every storage water heater. It opens automatically if water in the tank exceeds 210°F or 150 psi, dumping hot water through a discharge tube to prevent the tank from rupturing. It is the single most important safety device on a water heater.
What it is
The T&P combines two safety mechanisms in one valve. A long temperature probe extends down into the tank’s hot zone; if the water reaches roughly 210°F (99°C), a wax or fluid-filled element opens the valve. A spring-loaded pressure mechanism opens it independently if internal pressure exceeds 150 psi (1035 kPa). Either trip dumps water out the side outlet and into a downward-pointing TPR discharge tube that terminates near the floor.
The valve is built to ASME/ANSI Z21.22 standard. Threads are typically 3/4″ NPT, with long-shank or short-shank versions to match the heater’s insulation thickness. A manual test lever on top lets you (or an inspector) verify the valve operates by lifting it briefly — water sprays into the discharge tube, and the lever should snap back and the drip should stop within a few minutes.
Without a working T&P, an out-of-control overheat — a stuck thermostat or failed ECO cutoff — can take the tank past the boiling point under pressure. The result is a steam explosion powerful enough to launch a 50-gallon tank through a roof. The valve exists to make that impossible.
Why it matters to a homeowner
Three things to know.
First, a T&P that drips is almost always telling you about a closed-system thermal expansion problem, not a failed valve. If your house has a pressure-reducing valve or a check valve at the meter — common in newer WA construction — water can’t push back into the main when the heater warms up, and pressure climbs past 150 psi every time the burner fires. The drip is the T&P doing its job. The fix is usually a new or recharged expansion tank, not a new T&P.
Second, the test lever is for you. Manufacturer recommendation is to lift it every six months. If the valve won’t reseat after the test, it needs replacement — but skipping the test because “it might leak” isn’t a strategy, it’s avoiding the safety check the valve exists to allow.
Third, when a plumber tells you the T&P is leaking and you need a new water heater, push back. A bad T&P is a $30 part. A bad expansion tank is $50. Replacing a working heater for a dripping relief valve is the wrong fix.
When you’ll encounter this term
- The discharge tube near your water heater is dripping or staining the floor.
- A home inspector flags “T&P never tested” or “T&P won’t reseat” in a sale report.
- A plumber installing a PRV mentions you now need an expansion tank “because of the T&P.”
- You’re permitting a water heater swap and the inspector checks the T&P discharge termination.
Common variants and what a T&P is not
- Long-shank vs. short-shank. The probe length must match how thick the heater’s insulation is. Long-shank for thick-foam premium tanks, short-shank for older or thinner-walled heaters.
- T&P vs. PRV. The T&P is on the heater itself and protects the tank. A pressure-reducing valve (PRV) is on the cold supply at the building entry and limits incoming municipal pressure. Different jobs.
- T&P vs. dedicated T-only or P-only. Combined T&P is standard residential. Commercial and high-pressure systems sometimes use separate temperature-only and pressure-only valves on different ports.
Common failure modes
- Drips intermittently after a hot shower. Closed-system thermal expansion. Verify your expansion tank works (tap the tank — hollow on top, solid on bottom is good; all-solid means the bladder is waterlogged).
- Drips constantly. Sediment or scale fouling the seat. Replace the valve.
- Won’t lift on the test lever. Seized closed — actively unsafe. Replace immediately.
- Lifts but won’t reseat. Weak spring or damaged seat. Replace.
- Capped or plugged. Whoever did this created an explosion hazard. Remove the cap and replace the valve. Never re-cap.
Washington note
WA’s mechanical and plumbing code (WAC 51-56) adopts the Uniform Plumbing Code with state amendments and requires a combination temperature and pressure relief valve on every storage-type water heater. The discharge tube must run downward, full-size (no reducing), with no shut-off valve and no threaded end, terminating within 6 inches of the floor or to an approved drain receptor.
The closed-system question is more common in WA than in many states because water utilities in Seattle, Tacoma, Bellevue, and most King County cities require a backflow preventer or PRV at the meter for new and retrofitted services. Any heater installed downstream of one of those devices needs an expansion tank, and a missing or failed expansion tank is the number-one cause of T&P drips in WA homes built or repiped in the last 20 years.
FAQ
Why is my T&P relief valve dripping?
Nine times out of ten, it’s closed-system thermal expansion — your house has a check valve or PRV at the meter, water can’t push back to the main when the heater warms up, and pressure climbs past 150 psi every cycle. Install or replace the expansion tank and the drip stops. If the drip continues with a known-good expansion tank, the valve seat is fouled and the valve itself needs replacing.
How often should I test the T&P valve?
Manufacturer recommendation is every six months — twice a year. Lift the test lever for one to three seconds with a bucket under the discharge, then release. The lever should snap back and the drip should stop within a few minutes. If the valve won’t reseat, replace it. Wear gloves and stand to the side; the spray is scalding.
Can I plug or cap a leaking T&P?
No. Never. Capping a T&P creates an explosion hazard — without a relief path, an overheating tank can rupture violently. If yours is dripping, diagnose the cause (almost always thermal expansion) and either fix the expansion tank or replace the valve. A capped T&P found during a sale inspection is a deal-breaker until corrected.