T&P Relief Weeps After a New PRV and Meter
You are called to a 1998 two-story house with a 50 gal gas water heater. The city recently replaced the meter and installed a new 3/4 in PRV with a built-in backflow preventer set to 65 psi. Two weeks later the T&P valve began dripping, mostly after the burner runs. The homeowner replaced the T&P valve once; it still drips. A gauge on the water heater drain reads 70 psi cold and climbs to 135 psi after a 20 minute burner cycle, then slowly falls. Street pressure is 110 psi. There is no expansion tank. The heater has a 3/4 in cold inlet with a heat trap and a 1/2 in T&P discharge piped to within 6 in of the floor. The T&P is rated 150 psi/210°F. The thermostat is set to 140°F.
1. Diagnose the cause of the dripping T&P valve. Why did it start after the PRV installation, and what do the gauge readings tell you?
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The new PRV with a backflow preventer creates a closed system. When the burner heats the water, thermal expansion cannot push water back into the main, so pressure rises. The gauge going from 70 to 135 psi, a 65 psi rise, is the signature of thermal expansion, not a bad T&P valve. The T&P is rated 150 psi, so it should not open at 135 psi, but a worn or weeping valve can drip below its rating; the real defect is the missing expansion control. The 140°F thermostat increases the expansion. Confirm that the PRV is holding and that no bypass is open.
2. What correction does the code require, and how would you size and install the expansion tank?
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The plumbing code requires a means to control thermal expansion in a closed system, typically a listed expansion tank on the cold side between the PRV/backflow preventer and the water heater. Size the tank from the manufacturer's chart using system volume, maximum temperature rise, and precharge pressure. A 50 gal heater in a typical house often needs a 2 to 4.5 gal tank, but the chart governs. Install the tank vertically or as listed, support it, and set the precharge to the system static pressure. Do not cap or plug the T&P valve. Keep the discharge pipe full size and terminate it safely.
3. The drip continues after you install a tank. What would you check next?
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Check the expansion tank bladder and precharge; a failed tank is waterlogged and cannot absorb expansion. Verify the tank size and that it is connected to the cold side and not isolated by a check valve. Check for a stuck PRV or check valve, a recirculation loop or other heat source adding expansion, a thermostat stuck on, scale or sediment causing overheating, and gauge calibration. Verify the T&P rating and seat. If pressure still exceeds the tank's capability, add a second tank or another approved relief method per code. Never plug the T&P valve.