Six-Storey Apartment Pressure-Reducing Valve and Thermal Expansion
A six-storey, 24-unit apartment building is supplied by a 2 in. copper domestic water service from a municipal main at 690 kPa. A pressure-reducing valve at the entry is set to 350 kPa. Six weeks ago, building maintenance replaced the original PRV with the same model after a leak; no permit was posted. Since then, the superintendent reports: static pressure at the basement hose bib is 540 kPa; a top-floor unit gauge reads 310 kPa; the ground-floor water heater T&P relief valve weeps during recovery; and flushometer valves on the second floor chatter. There is no expansion tank on the domestic system, and the new PRV has no bypass. The building has a closed system because the PRV also contains an integral check seat. A plumber has been asked to determine whether the PRV is defective, whether thermal expansion is contributing, and what must be corrected before the next inspection.
1. Evaluate the pressure readings and determine the most likely reason the basement pressure exceeds the PRV setpoint. Show the expected elevation effect and explain what you would check.
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Expected hydrostatic loss is about 3 m per storey × 5 storeys = 15 m; 15 m × 9.81 kPa/m ≈ 147 kPa. If the basement were correctly limited to 350 kPa, the top floor would be about 203 kPa, not 310 kPa. The basement reading of 540 kPa is therefore roughly 190 kPa above setpoint, which points to the PRV not regulating rather than elevation alone. Possible causes include debris on the seat, a failed diaphragm or spring, wrong orientation, a missing internal check, an open bypass, or an inaccurate gauge. The top-floor reading being lower than the simple elevation prediction (540 − 147 ≈ 393 kPa) suggests gauge error or additional dynamic losses, but it does not explain away the high basement pressure. I would verify with a calibrated gauge at the PRV outlet, inspect the strainer and flow arrow, compare the model and setpoint, and test both static and flowing pressure. Flushometer chatter is consistent with overpressure. Repair or replace the PRV; do not simply adjust the top-floor fixtures.
2. Explain the T&P relief discharge and the requirements for expansion control in this closed system.
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The PRV with an integral check seat isolates the building from the main, making the domestic system closed. When the water heater recovers, thermal expansion has nowhere to go, so pressure rises until the T&P relief opens. The T&P is a safety device, not a control; repeated weeping can damage the valve and its discharge line. Corrective work should include a listed expansion tank on the cold-water side downstream of the PRV/check, sized for system volume, maximum supply pressure, and temperature. The tank precharge should be set to the system static pressure. The T&P rating must match the system, and the discharge pipe must be full-size, terminate safely, and remain unblocked. The PRV must also be corrected so the system is not overpressurized. Record static pressure before and after, and check for any other closed valves or backflow devices that could create additional closed sections.
3. List the corrective actions and documentation needed before the system is returned to service.
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Corrective actions: obtain a permit where required; isolate and drain; replace or repair the PRV; install a strainer if missing; install an expansion tank; replace any damaged T&P valve; flush; pressure-test; disinfect if potable work was opened; restore service; measure static and flowing pressure at high and low points; and verify relief discharge. Documentation should include the permit number, PRV model and setpoint, test-gauge calibration, before-and-after pressures, expansion-tank size and precharge, photographs, inspection sign-off, and owner notification. Also check flushometer valves for damage from overpressure and monitor the system for several days. The key judgement is that the PRV and expansion control are separate functions; fixing one does not fix the other.