Apartment Building Pressure Control, Thermal Expansion, and Relief Discharge
A 40-unit apartment building has a 2-inch domestic water service entering at 95 psi. A reduced-pressure-principle backflow preventer and a pressure-reducing valve set at 60 psi are installed at the meter. Each apartment has a 50-gallon gas water heater; the building also has a 200-gallon storage tank and a recirculation loop. The maintenance technician reports that T&P relief valves on several heaters drip only when the building is occupied in the evening and the recirculation pump runs. There is no expansion tank at the heaters or storage tank. The owner wants to raise the PRV setting to 80 psi to improve top-floor shower pressure and install a 3/4-inch T&P discharge line terminating in a floor drain with a threaded cap. The building is a closed system because of the backflow preventer and PRV.
1. Explain the likely cause of the dripping T&P relief valves and the code-required correction.
Read the worked answer
The building is a closed system because the reduced-pressure-principle backflow preventer and the pressure-reducing valve prevent expanded water from pushing back into the street main. When the water heaters heat water, the water expands and the pressure rises. Without a thermal expansion tank or other approved expansion control, the T&P relief valve becomes the pressure relief point and drips. The evening occupancy and recirculation pump pattern fits this because more hot water is being generated and circulated when demand is high.
The correction is to install an approved diaphragm-type thermal expansion tank on the cold-water side between the backflow preventer or PRV and the water heater, sized for the system volume and the maximum temperature and pressure. The tank precharge must be set to match the system pressure. The T&P valve must remain in place and must not be plugged. The installer should also verify that the PRV is not bypassing and that the relief valve rating matches the heater. Adding a larger T&P valve or capping the discharge would not correct the expansion problem.
2. Evaluate the owner's proposal to raise the PRV setting to 80 psi and to install the described T&P discharge line.
Read the worked answer
A PRV is required where pressure exceeds 80 psi, so setting the PRV at 80 psi is at the code threshold and leaves no margin for pressure fluctuations. It also may not solve the top-floor pressure complaint if the real problem is friction loss, an undersized riser, or an unbalanced recirculation loop. The better approach is to measure static and residual pressure at the top floor and size the piping or adjust the recirculation before raising the PRV. If the setting is adjusted, it must not exceed the limit and must remain compatible with fixture and appliance ratings.
The T&P discharge proposal is not acceptable as described. The discharge pipe must be the same size as the valve outlet, not reduced to 3/4 inch if the valve outlet is larger. It must terminate safely, typically not more than 6 inches above a floor drain or outside, with no trap, no threaded cap, and no valve. The pipe material must be rated for the relief temperature. A threaded cap on the discharge is a serious hazard because it can block relief and cause a tank or piping failure.
3. What checks and tests should be completed before the expansion correction is accepted?
Read the worked answer
Verify the expansion tank size against the manufacturer's chart using the system volume, maximum temperature, supply pressure, and relief setting. Check the precharge with the tank isolated and the system depressurized, then confirm the tank is connected to the cold-water side without a check valve between the tank and the heater. Operate the recirculation system through a heating cycle and observe the T&P discharge; it should remain dry after the expansion tank is installed. Test the backflow preventer and confirm the PRV outlet pressure under flow, not just static. Check that every T&P valve has a full-size, properly terminated discharge line and that no discharge is capped or reduced. Document the pressure readings and tank precharge so the final inspection can verify the correction rather than just looking at the new tank.