Why Your Water Heater Relief Valve Drips

A T&P valve is a spring-loaded relief device threaded into the top or upper side of a water heater tank, with a discharge pipe routed down toward the floor. It opens automatically if either tank pressure exceeds its rated setting, typically 150 psi, or tank temperature exceeds its rated setting, typically 210 F, whichever happens first. Under those conditions a sealed tank can otherwise build pressure with nowhere to go, and the ASME requirements this valve is built to exist specifically to prevent that outcome.

A valve that opens and closes on its own during ordinary operation, releasing a small amount of water each time, is discharging exactly as designed in response to pressure or temperature briefly crossing its threshold. That is not the valve malfunctioning; it is the valve doing its one job. The question worth answering is what is pushing pressure or temperature that high in the first place.

If your water supply is a closed system, meaning a check valve, pressure-reducing valve, or backflow preventer sits somewhere upstream and prevents water from pushing back out toward the street main, then heated water inside the tank has nowhere to expand into except the T&P valve. Water expands measurably as it heats from a cold fill temperature toward a typical 120 to 140 F setpoint, and in a closed system with no expansion tank, that expansion raises tank pressure directly. If that pressure rise crosses the valve rated setting, even briefly at the peak of a heating cycle, the valve opens and drips.

This is by a wide margin the most common cause of an otherwise-fine valve dripping on a schedule tied to heating cycles, particularly right after recovery from a large hot water draw. The fix is a correctly sized, correctly pre-charged expansion tank on the cold inlet, never removing or restricting the valve.

If static supply pressure, meaning pressure measured at a hose bibb with no fixtures running, already sits close to or above the valve rated setting before the water heater has even fired, an expansion tank cannot fully solve the problem because there is too little headroom left for expansion to work with. High static pressure is often a municipal supply issue, a failed or absent pressure-reducing valve on the main, or elevation and distance from a booster pump. In this case the correct fix is a pressure-reducing valve installed by a licensed plumber to bring static supply pressure down into a normal residential range, after which an expansion tank can do its job on the remaining thermal expansion.

A T&P valve is a mechanical device with a spring and a seat, and like any mechanical device it can fail, most often from mineral scale building up on the seat and preventing a full reseal after it opens, or from age simply wearing the spring past its rated tolerance. A valve that drips continuously rather than in response to a heating cycle, or one that drips even when static pressure and expansion tank pre-charge both check out normal, is more likely a failed valve than an upstream pressure problem. Manufacturers do not publish a fixed replacement interval; a valve is replaced when it fails a manual test or shows continuous discharge that pressure and expansion checks cannot explain.

Never cap, plug, remove, or install a shutoff valve anywhere in the T&P discharge line. A capped or removed relief valve does not make the underlying pressure or temperature problem go away, it removes the one device standing between a sealed tank under rising pressure and a tank that has no way to relieve it, which is a genuine hazard. The discharge pipe itself should run to within six inches of the floor, be full size with no reduction, have no threads on its open end, and have no upward run anywhere in its path, since a T&P valve is a safety device and its discharge path is part of that safety design, not an incidental detail.

Buy the part that matches the cause you have confirmed with a gauge, not the part that is easiest to order first.