Water heater relief valve keeps dripping: worn valve, high pressure or a missing expansion tank

The owner, writing in Reddit's r/Plumbing forum, lifted the lever on the temperature and pressure (T&P) valve to flush it, saw dirt come out, and still has a slow drip from the discharge pipe. A. O. Smith's technical bulletin on leaking relief valves reads the symptom this way: slow weeping or dribbling means the valve is opening on pressure, while a sudden rush of very hot water means it opened on temperature, which the company blames only on a failed thermostat.

The valve's temperature rating is normally 210°F, and its pressure rating should not exceed the tank's working pressure, generally 150 psi, according to the bulletin. Water around the threads calls for resealing the valve; a leak at the fitting welded to the tank means the heater has to be replaced.

Thermal expansion is the usual reason for a pressure drip. Between 40°F and 120°F, water expands by roughly 2%, according to Jaspector's summary of the 2024 International Residential Code. When a pressure-reducing valve, check valve or backflow preventer stops that water from flowing back to the main, pressure builds until the T&P valve lets some out. Section P2903.4 requires a way to control that pressure, in practice an expansion tank on the cold inlet with its air charge set to the home's static pressure. The code also caps static pressure at 80 psi and requires a reducing valve above that.

If pressure checks out, the valve itself is next. Lennox says scale and sediment caught in the valve seat can keep it dripping after pressure drops, and that a valve that won't seat again or shows corrosion should be replaced instead of cleaned. A waterlogged expansion tank, which sounds solid all the way up when tapped, also lets pressure climb.