Water Heater T&P Relief Dripping After Replacement
A 50 gal gas storage water heater is replaced in a house with a 1 in. pressure-reducing valve set to 60 psi and a backflow preventer at the meter. There is no expansion tank. Two weeks later the T&P relief valve drips after each heating cycle; a gauge on the drain valve shows pressure rising from 60 to 125 psi. The homeowner suggests capping the T&P discharge because the floor drain is across the room and the existing pipe runs 12 ft horizontally with a trap before terminating near the drain.
1. Diagnose the cause of the dripping T&P valve and the pressure rise.
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The system is closed by the PRV and the backflow preventer. When the water heater heats water, thermal expansion has nowhere to go, so pressure rises from 60 psi to 125 psi and the T&P valve opens. The absence of an expansion tank is the likely cause. A failed PRV or a failed T&P valve could also contribute, but the repeated rise after each heating cycle points to thermal expansion.
2. Evaluate the existing T&P discharge piping and the homeowner's proposal to cap it.
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The T&P discharge must remain full size, must not be capped or plugged, must not be trapped, and must flow by gravity to a safe termination. A 12 ft horizontal run with a trap is improper because the trap can hold water and block discharge, and the pipe may not drain fully. The termination should be at least 6 in. above a floor drain with an air gap or outside the building, and the pipe should be as short and straight as practicable. Capping the T&P valve is never acceptable because it defeats the safety device.
3. What is the correct repair, and what should be checked after the repair?
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Install a properly sized thermal expansion tank on the cold-water side of the water heater, sized for the system volume and the PRV setting. Remove the trap and reroute the T&P discharge to a proper air-gap termination or outside. Check the PRV for proper operation and verify the T&P valve is not damaged; replace it if it has been leaking or tested improperly. After the repair, verify that pressure stays below the relief setting through a heating cycle and that no discharge occurs.