What it is

The T&P (temperature-and-pressure relief) valve is the one device standing between a water heater and a catastrophic tank rupture. It's a spring-loaded valve threaded into the top (or upper side) of every storage tank. If the water gets too hot or the pressure gets too high, it opens and dumps water to relieve the condition. It's the last line of defense against a runaway tank, and it is non-negotiable on every install.

If you understand nothing else about water heater safety, understand this: a tank full of water heated well past boiling, held together only by the tank shell, contains an enormous amount of stored energy. Let that shell fail and the superheated water flashes to steam instantly, and the tank becomes a rocket. The T&P valve exists to make sure the water never gets there.

How it works

The valve has two independent triggers:

  • Pressure element: a spring set to open at a fixed pressure — almost always 150 psi on residential heaters. If tank pressure climbs to 150 psi (from thermal expansion, a stuck heating control, or a closed system with no expansion tank), the valve cracks open and relieves it.
  • Temperature element: a probe (often a wax/thermostatic element) that reaches into the top of the tank water and opens the valve if the water hits about 210°F — just below boiling at atmospheric pressure. This protects against the real danger: a heater whose thermostat or gas control has failed and is heating without limit.

Either trigger opens the valve. The temperature side is the critical one, because that's what prevents the steam-explosion (BLEVE-type) scenario. A tank that overheats with a stuck control and a blocked or missing T&P is exactly how water heaters violently fail.

In the field

What you actually do with T&P valves:

  1. Verify it's there, rated correctly, and oriented right. The valve's relief temperature/pressure (210°F / 150 psi) must be at or below the tank's rated working pressure. The probe must be in the water, not screwed into an empty bushing.
  1. Check the discharge pipe. This is where techs and handymen fail constantly. The discharge line must be the same diameter as the valve outlet (typically 3/4"), run downhill the whole way, be made of an approved material rated for the temperature, terminate within a safe distance of the floor or to an approved point, and have no valve, no cap, no threads on the end, and no reduction. It can't trap water and it can't be plumbed where a discharge would scald someone or go unnoticed.
  1. Test it (carefully). Lifting the test lever should produce a strong flow of water and then reseat. On an old valve, lifting it can keep it from reseating (the seat is mineral-crudded) — be ready to replace it. Many techs just replace a 5+ year old valve rather than risk it.
  1. Diagnose weeping/dumping — it's almost never the valve "going bad" on its own. See faults below.

Normal values & targets

  • Standard residential rating: 150 psi pressure, 210°F temperature relief.
  • Discharge pipe: same size as the outlet (typ. 3/4"), no reduction, no threads on the terminal end, pitched continuously downward, ending an approved short distance above the floor/drain (often within ~6 inches of the floor) or to an approved location.
  • Normal supply pressure: street/well pressure should sit well under 80 psi; above that, code wants a pressure-reducing valve. A T&P set at 150 psi should never relieve on normal pressure.
  • Expected service life: these are cheap; replacing on water-heater change-out and any time it weeps is standard.

Common faults & what they mean

  • Valve drips/weeps intermittently. Usually thermal expansion in a closed system — every time the burner/element heats the water it expands, pressure spikes past 150 psi, and the valve relieves a little. The fix is an expansion tank, not a new T&P. (A check valve, PRV, or backflow preventer "closes" the system; without an expansion tank, the T&P becomes the pressure relief by default.)
  • Valve weeps constantly / streams. Supply pressure too high (failed PRV), or the valve seat is fouled and won't reseat. Check static pressure first; replace the valve if pressure is normal.
  • Valve dumps hot water / lets go hard. The temperature side opened — the water actually got to ~210°F. This is a serious finding: the heating control failed and the tank overheated. Find and fix the runaway (stuck gas control, welded thermostat, grounded element) — the T&P just saved the house.
  • No drip but you find it plugged/capped. Someone "fixed" the dripping by capping it. This is dangerous — the tank now has no relief. Remove the cap, fix the real cause (expansion tank/PRV), install a proper discharge line.

Tech tips & gotchas

  • A dripping T&P is a symptom, not the disease. Nine times out of ten it's thermal expansion needing an expansion tank, or high street pressure needing a PRV. Swapping the valve without fixing the cause just means it drips again next week — or worse, the homeowner caps it.
  • Never, ever cap, plug, or valve off a T&P discharge. If you find one capped, that's a red-tag-level safety issue. The whole point is unobstructed relief.
  • The temperature relief is the one that matters most. Pressure relief handles nuisance expansion; temperature relief prevents the explosion. Make sure the probe is actually in the tank water.
  • A T&P that won't reseat after you test it is telling you it's done — carry spares and just replace it.
  • On older tanks, test gently and be ready for it not to reseat. Don't test one you're not prepared to replace.

Safety / code notes

  • T&P valves and their discharge piping are required and detailed by the plumbing code water-heater/relief provisions (IPC §504 covers water-heater relief and discharge piping) and the appliance/valve listing. Cite the section; follow the listing for rating and installation.
  • The discharge line rules (full size, downhill, approved material, no valve/cap/threaded end, approved termination) exist so relief is never blocked and a discharge never scalds anyone — follow them exactly.
  • Thermal expansion control (expansion tank) is required on closed systems by the plumbing code; it's the correct fix for a T&P relieving on expansion, and it protects the whole plumbing system, not just the heater.
  • A capped, plugged, or missing T&P, or one whose probe isn't in the water, is a life-safety hazard (steam-explosion risk) — correct it before leaving and recommend the homeowner never bypass it.