The relief valve is the heater's last line of defense against dangerous temperature or pressure. Dripping is a symptom worth fixing, not a valve to block.
Safety first
Never plug, cap or remove the relief valve or its discharge line, and never put a shutoff valve between it and the tank. Discharge water can be extremely hot.
Common causes
- Thermal expansion: heated water expands, and in a closed system (check valve, backflow preventer or pressure regulator) the pressure has nowhere to go.
- Household water pressure that is too high. Relief valves are typically set to open at 150 psi, and 80 psi is the residential maximum under the plumbing code.
- A worn valve that no longer reseats.
The fix
A.O. Smith says a properly sized expansion tank should be installed on closed systems, and a pressure-reducing valve where pressure is excessive. California Plumbing Code section 608.3 requires thermal expansion control on closed systems. Many San Diego homes have a pressure regulator, which closes the system.
Testing the valve
A.O. Smith recommends operating the valve by hand once a year, with no one near the outlet. If it will not reseat afterward, the cold inlet should be closed, the tank drained and the valve replaced, which a plumber can do.
General information, not professional or legal advice; follow your manufacturer's manual, and your installer and local permit office have the final word. Sources:A.O. Smith residential electric water heater manual (185762); State Water Heaters, High water pressure, thermal expansion and T&P valve dripping; Bradford White installation & operating manual; California Plumbing Code sections 507.2, 507.5 and 608.3 (as summarized in a city water heater handout).
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