Mixed-Use Water Service: PRV, Expansion, and Irrigation Backflow
A four-story mixed-use building has a 1-1/2-inch copper water service with a static pressure of 115 psi. A pressure-reducing valve at the basement main is set to 55 psi. An 80-gallon commercial water heater is installed downstream of a backflow preventer and the PRV; there is no thermal-expansion tank. The T&P relief valve has a 3/4-inch outlet, but the discharge is reduced to 1/2-inch copper with a reducing bushing, passes through a wall, and terminates 5 feet above the floor in a janitor closet. An irrigation system is connected to the domestic service upstream of the PRV and backflow preventer, with no backflow assembly. The contractor requests rough inspection before the water heater is enclosed.
1. Identify the water-supply and T&P discharge violations and state the corrections.
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The service pressure of 115 psi exceeds the usual threshold at which a pressure-reducing valve is required, and a PRV is present and set to 55 psi, so that part is generally acceptable. The problem is the closed system created downstream of the PRV and backflow preventer: without thermal-expansion control, heating the 80-gallon tank will raise pressure and may discharge the T&P valve or damage components. An expansion tank or other approved expansion control is required.
The T&P discharge is noncompliant. It is reduced from 3/4 inch to 1/2 inch, which violates the full-size discharge requirement. It must be full size, must not be reduced or trapped, and must terminate within the usual 6-to-24-inch range above the floor or an approved receptor. Terminating 5 feet above the floor is too high and would not safely direct a discharge.
The irrigation connection is upstream of the PRV and backflow preventer, so it has no backflow protection. A separate approved backflow assembly appropriate to the hazard is required on the irrigation branch. The inspector should require corrections before enclosure and before the irrigation line is placed in service.
2. Why is thermal expansion control required here, and what else should the inspector verify about the water heater installation?
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The PRV and backflow preventer isolate the building water system from the public main, creating a closed system. When the water heater raises the water temperature, expanded water has nowhere to go, so pressure rises. The T&P valve is a safety device, not a normal expansion control. Repeated relief discharge can damage the valve and piping.
The inspector should verify that the expansion tank is sized for the system volume and temperature and installed on the cold-water side downstream of the backflow preventer/PRV in accordance with the manufacturer. The inspector should also check the T&P valve rating, that no valve is installed between the heater and the T&P valve, that the discharge is full size and properly terminated, and that the relief line does not create a trap or hidden discharge.
Other checks include the water heater venting and combustion air, a drain pan and drain where required, seismic strapping if the jurisdiction requires it, and accessibility for service. The expansion tank correction and T&P correction should be completed before the water heater is enclosed.
3. How should the irrigation connection be protected, and what inspection sequence should be required before enclosure?
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The irrigation branch must have an approved backflow assembly appropriate for the degree of hazard. Because irrigation systems can be subject to contamination and the connection is upstream of the domestic PRV/backflow protection, the inspector should not accept an unprotected tee. The assembly type and protection strategy should follow the adopted UPC and the local water purveyor, often an RPZ or other approved assembly depending on the hazard.
The inspection sequence should be: verify approved plans and permits; inspect the service, PRV, backflow assembly, and irrigation branch before concealment; require installation and testing of the backflow assembly by an approved tester; correct the water-heater expansion and T&P discharge; pressure-test the domestic water piping; and inspect the water heater before enclosure. Final approval should be withheld until test reports and corrections are documented.