T&P Valve (Temperature & Pressure Relief)
The Temperature and Pressure Relief Valve (T&P valve) is a code-required safety device installed on every residential and commercial water heater. Its job is simple but critical: prevent the tank from rupturing or exploding by venting steam and hot water before internal conditions reach failure point. The valve is factory-set to open when water temperature exceeds 210 degrees Fahrenheit or when internal pressure exceeds 150 pounds per square inch (psi). Without a functioning T&P valve, a stuck thermostat or runaway heating element can turn a water heater into a high-pressure projectile.
Identification: The T&P valve is a brass fixture mounted on the top or upper side of the water heater tank, with a metal lever or handle on top and a discharge tube (usually copper or CPVC) running down the side of the tank to within 6 inches of the floor. The discharge tube must terminate in an approved location and cannot be capped, threaded, or reduced in diameter. Markings stamped into the brass body show the relief settings (typically 150 psi / 210 F) and the certification standard, most commonly ANSI Z21.22 or ASSE 1004.
Lifespan and failure modes: Manufacturers recommend testing the lever annually by lifting it briefly to confirm water discharges and the valve reseats without dripping. Common failures include mineral buildup that prevents the valve from seating (causing constant slow drips), corrosion of the spring assembly, and complete seizure from years of inactivity. A T&P valve that drips intermittently is often signaling a real problem upstream, such as excessive system pressure above 80 psi, thermal expansion in a closed system without an expansion tank, or a thermostat stuck in the on position. Replacing the valve without diagnosing the root cause typically results in the new valve dripping within weeks.
Replacement framing: The valve itself runs $40 to $80 at supply houses. Professional replacement labor typically falls between $150 and $250 for a straightforward swap, more if the discharge piping must be reworked or if the valve seat in the tank is corroded. Replacement is required immediately if the valve is leaking continuously, fails the annual lever test, or shows visible corrosion. The water heater must be powered down and partially drained below the valve port before removal.
Code reference: Installation requirements are governed by the International Plumbing Code (IPC) Section 504 and the Uniform Plumbing Code (UPC) Section 504, both of which mandate a T&P valve on every storage water heater. ASSE Standard 1003 covers performance and testing criteria for the valves themselves. Local jurisdictions may add specific discharge pipe routing rules, including prohibitions against terminating into a closed drain or above grade in freeze-prone climates.