High-Rise Apartment Pressure Drop After RPZ and PRV Installation
A 24-unit, three-story apartment building gets a new 1¼-inch water service, an RPZ backflow assembly at the meter, and a PRV set to 60 psi. The city static pressure is 110 psi. The building has tankless water heaters on each floor and a recirculation pump. At the initial walkthrough, the lower-floor hose bib reads 85 psi, the top-floor shower flows only 38 psi, and the tankless units occasionally shut down with a pressure fault. The installing contractor says the PRV is ‘set at 60’ and blames the tenant fixtures. The plumber notices no thermal expansion tank on the domestic system, the RPZ test tag is blank, and the PRV has a strainer that has never been cleaned. The plumber must determine whether the problem is pressure, flow, or both and what code and safety corrections are needed.
1. The PRV is set at 60 psi, yet the lower floor reads 85 psi and the top floor 38 psi. Give a reasoned diagnosis.
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The readings are inconsistent with a properly functioning PRV set at 60 psi. A 60-psi downstream setting should hold the lower floor near 60 psi; 85 psi suggests the PRV is bypassing, installed backward, fouled by a clogged strainer, or has a failed seat. The top floor should lose about 4.3 psi per 10 ft of elevation, so 30 ft accounts for roughly 13 psi, not 47 psi. The 38-psi reading therefore indicates additional friction loss, a partially closed valve, a clogged aerator or fixture, or a recirculation problem. The plumber should verify with calibrated gauges at the PRV inlet and outlet, clean or replace the strainer, confirm flow direction, and measure pressure while water is flowing. The tankless pressure faults suggest the minimum inlet pressure is not being maintained during simultaneous demand.
2. What backflow and thermal-expansion requirements apply, and what is the risk of leaving them unresolved?
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An RPZ at the service creates a closed domestic system, and the PRV adds another point of closure. A closed system needs a properly sized thermal expansion tank on the domestic water system, plus the water heater’s own temperature and pressure relief protection. Without expansion control, heating water can raise pressure above the 80-psi fixture limit and stress tanks, fittings, and tankless heat exchangers. The RPZ must be tested by a certified backflow tester at installation and at the required interval, and the blank tag must be completed. The plumber should also verify that the tankless units’ relief valves and minimum/maximum pressure requirements are met. Leaving the RPZ untested and the expansion tank missing creates a cross-connection risk and a pressure-safety risk, and it may cause repeated tankless shutdowns or water damage.
3. What tests and corrections should the plumber specify before sign-off?
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The plumber should specify calibrated static and flowing pressure tests at the service, each floor, and the farthest fixture; a flow test at the top floor; cleaning or replacement of the PRV strainer; and repair or replacement of the PRV if it cannot hold 60 psi downstream. The RPZ must be tested and tagged by a certified tester. A thermal expansion tank sized to the system volume and heater capacity must be installed, and the tankless units’ pressure requirements and relief piping must be verified. The recirculation pump should be checked for excessive pressure drop or a stuck check valve. Only after these corrections should the contractor sign off. The report should include gauge readings, RPZ test results, expansion-tank precharge, and confirmation that the top-floor fixture meets minimum pressure during simultaneous demand.