Restaurant Water Heater Relief Valve Dripping After Backflow Prevention
A 90-seat restaurant has a 1-inch copper water service at 85 psi static pressure. A pressure-reducing valve at the meter is set to 50 psi, and a reduced-pressure-principle backflow preventer was installed downstream of the meter last year. The 100-gallon gas-fired water heater has a 3/4-inch T&P valve piped in 3/4-inch copper to a floor drain; the discharge terminates 8 inches above the drain. Since the backflow preventer was installed, the T&P valve drips continuously, and a gauge at the heater's drain valve reads 145 psi after a busy lunch service. The restaurant is adding a carbonator for fountain drinks and a coffee urn. The owner wants to remove the T&P valve because "it keeps leaking" and install a larger expansion tank on the cold main. The local inspector has flagged the carbonator connection and the water heater relief piping. The building is on a public water supply and has no booster pump.
1. Diagnose the cause of the T&P valve drip and the 145 psi reading, and state the correct expansion-control correction.
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The RPBP and the PRV isolate the water heater from the public main, creating a closed system. When the burner heats 100 gallons of water, thermal expansion raises pressure because water cannot push back through the backflow preventer. A gauge reading 145 psi while the PRV is set at 50 psi is strong evidence of thermal expansion, not a normal supply-pressure condition. The T&P valve may be doing its job by relieving pressure, or it may be weakened and failing to reseat; either way, removing it is prohibited. Install a listed thermal expansion tank on the cold-water line between the backflow preventer/PRV and the heater. The tank must be sized for the system volume, the maximum heater temperature, and the supply pressure, and its precharge must match the cold static pressure. Do not simply add a larger tank on the cold main upstream of the backflow preventer; it must be downstream where it can absorb expansion. After the tank is installed, test the T&P valve and replace it if it does not reseat.
2. What backflow protection is required for the new carbonator and coffee urn, and how should the connections be arranged?
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The carbonator is a high-hazard connection because carbon dioxide can be forced back into the potable water line. It requires an air gap or a reduced-pressure-principle backflow preventer, depending on the local code and the equipment listing; an air gap is the more protective choice. The carbonator should not be connected with a simple check valve or a hose-bib vacuum breaker alone. The coffee urn is a lower-hazard connection but still requires an approved backflow preventer or air gap if it is directly connected to the water supply. The inspector's flag should be resolved by providing an air gap at the carbonator fill line and a listed backflow preventer for the urn, with both devices accessible for testing and maintenance. The RPBP already serving the building should be tested annually, and the new devices must not be installed in a location where they can be submerged or where discharge from the relief port can cause damage.
3. The owner proposes to remove the T&P valve and rely on the expansion tank. Explain why that is unacceptable and what relief-piping corrections are needed.
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The T&P valve is the final safety device on the heater and cannot be removed, plugged, or replaced with a lower-rated valve. The expansion tank controls normal expansion, but it does not protect against a failed thermostat, a stuck burner, or an overpressure event. The relief valve must be rated for the heater's maximum working pressure and temperature, and its discharge must remain full size. The existing 3/4-inch discharge is correctly sized but terminates 8 inches above the drain, which is acceptable only if the code-required maximum above the floor or receptor is met; verify the local limit, commonly 6 inches. The discharge must flow by gravity, have no traps, no threaded end, and no reduction, and it must terminate where hot water or steam cannot injure someone. If the valve is leaking because it is damaged, replace it with a matching listed valve. The expansion tank and the T&P valve work together; neither replaces the other.