A T&P relief valve, short for temperature and pressure relief valve, is a spring-loaded safety device on a water heater that opens automatically if tank water reaches about 210°F or pressure reaches about 150 psi. Required by plumbing codes and built to the ANSI Z21.22 standard, it discharges through a pipe to prevent a tank explosion.
Why it matters to you
It is the last line of defense against a tank failing violently, so a valve that drips, weeps, or has been capped is a safety problem, not a nuisance.
Practical guidance
Never cap, plug, or remove a T&P relief valve or its discharge pipe. Test it once a year by briefly lifting the lever with a bucket under the pipe and hands clear of the hot water; if it keeps dripping afterward or will not open, have the valve replaced and the water pressure checked.