T&P Dripping After a Meter Backflow Preventer
A 12-story condominium has a 6 in. domestic service at 105 psi street pressure. A PRV in the basement is set to 55 psi. A backflow preventer was installed at the meter last year, and a check valve is on the cold-water inlet of each apartment water heater. On the 11th floor, a 50 gal gas water heater's T&P valve began dripping after the new backflow preventer was installed. The static pressure at the apartment hose bibb is 48 psi. The building engineer says the PRV is bad because the T&P is leaking and proposes raising the PRV setting to 70 psi to 'push water up.' The owner wants to replace the T&P valve only. The water heater is in a closet with a 1 in. copper drain pan; the T&P discharge is piped in 1/2 in. copper to a 12 in. air gap over a floor drain. The apartment is on the top floor.
1. Diagnose the cause of the T&P discharge and evaluate the engineer's proposed fix.
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The likely cause is thermal expansion, not a failed PRV. The new backflow preventer at the meter and the check valve at the heater create a closed system. When the burner heats water, the water expands and pressure rises until the T&P relief opens. At 48 psi static the system is not over the PRV setting, but the T&P sees the expansion pressure. Replacing the T&P alone will not fix it, and raising the PRV to 70 psi makes the expansion worse. Install a listed expansion tank on the cold-water side between the check valve and the heater, sized for the system volume and the relief setting, with precharge matched to the supply pressure. Confirm the T&P is rated for the heater and that the relief setting is not exceeded. Permit and inspection are required.
2. Is the existing T&P discharge piping acceptable? Explain what must be corrected.
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The 1/2 in. T&P discharge is likely undersized and must be corrected. The discharge pipe must be at least the size of the valve outlet, typically 3/4 in. for a 50 gal residential heater, and must run full size, by gravity, with no traps, no reducers, and no threaded end. It must terminate through an air gap to a floor drain, receptor, or outside, and within the required distance of the floor or receptor. The 12 in. air gap is good, but the 1/2 in. copper creates a restriction and may not carry full relief flow. The drain pan also needs a separate 1 in. discharge to a safe location. Correct the pipe size and termination before replacing the valve.