Home Safety
Every tank water heater has a temperature and pressure (T&P) relief valve that opens automatically if tank temperature or pressure runs too high. It must be present, working, and never capped — it's the tank's emergency safety relief.
Water expands when heated — and if a thermostat sticks and the burner won't shut off, the tank becomes a sealed vessel with rising pressure and superheated water. The T&P valve is the engineered failure point: it opens at set limits and dumps hot water safely through its discharge pipe instead of letting pressure build toward tank failure. It's a simple mechanical part doing one critical job.
California's plumbing code (CPC 608.5) spells out how the valve's outlet must be piped to do its job safely.
Occasional dripping usually means one of three things: system water pressure is too high (a failing pressure regulator — common in parts of our area with high street pressure), thermal expansion has nowhere to go (California's plumbing code requires an expansion tank on closed systems), or the valve itself is aging out. Each has a proper fix. The improper fix — capping the valve or plugging the discharge — converts a drip into a pressure vessel problem. It's one of the most dangerous shortcuts in residential plumbing.
The valve has a test lever, and Watts' T&P instructions call for operating it at least once a year — after confirming a discharge line is connected, since hot water will come out. It should discharge and then reseat cleanly. If no water flows, the manufacturer says the valve is inoperative: turn off the water heater and call a plumber. If it won't stop dripping after a test, it's done too — replacement is a routine repair. We test the T&P, check system pressure, and verify the discharge piping on every water heater maintenance visit; if your tank hasn't had that in years, it's a five-minute check that covers the highest-consequence failure the appliance has.
Very likely — that's the T&P discharge pipe doing its job or telling you about pressure. Occasional small discharges after heavy hot-water use suggest thermal expansion; steady dripping points at high pressure or a tired valve. Either way it's worth a check: the causes are fixable and valve replacement is routine.
No — never. Capping the T&P valve removes your water heater's only overpressure protection while the underlying problem continues. If the dripping bothers you, that's the signal to get the actual cause fixed (pressure regulator, expansion tank, or valve replacement), not to silence the messenger.
California's plumbing code (CPC 608.2) requires a pressure regulator where static water pressure exceeds 80 psi, with an expansion tank downstream of the regulator. Parts of LA & OC have high street pressure where a working regulator is essential — when regulators fail (they age like everything else), pressure climbs and every fixture, hose, and valve in the house feels it, T&P included. A gauge check takes minutes.
Sources
Last reviewed: 2026-09-30
Book Service
Real people answer around the clock. Tell us what's going on and we'll dispatch a licensed pro from the office nearest you.
