The Hotel Booster Pump and a Missing Thermal Expansion Control
A 14-story hotel is served by a 6-inch domestic service line with a reduced-pressure principle assembly (RPZ) installed immediately downstream of the city meter in a below-grade vault. The building has a private booster pump system set to maintain 85 psi at the top floor. During a renovation, the original pressure-reducing valve downstream of the RPZ was removed and the thermal expansion tank on the domestic water heater was disconnected because it interfered with new piping. Two months later, the water purveyor's cross-connection inspector finds the RPZ relief port discharging continuously at roughly 2 gpm, and the downstream gauge reads 118 psi during the night when the booster is off. The property manager argues the RPZ is defective and wants it replaced.
1. Identify the most probable cause of the continuous relief discharge and explain the hydraulic mechanism.
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The RPZ is likely functioning correctly. The relief port opens when the differential between the first and second check zones exceeds roughly 5 psi, or when the reduced-pressure zone rises above atmospheric.
Here the mechanism is thermal expansion backpressure, not a failed relief valve. The thermal expansion tank was removed and the downstream pressure-reducing valve was eliminated. The RPZ discharge port is a discharge, not a relief valve in the plumbing sense: it cannot relieve downstream pressure. Instead, when heated water expands against the closed second check, the downstream pressure rises until it exceeds the first-zone pressure. The differential across the second check reverses, the relief valve senses the rising zone pressure, and it dumps to drain.
The 118 psi night reading confirms downstream pressure above the normal 85 psi setpoint. The continuous 2 gpm discharge is the symptom; the missing expansion control is the cause. Replacing the RPZ would reproduce the failure on the new assembly.
2. Recommend corrective actions and explain why each is required for compliance and function.
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Restore thermal expansion control. Reinstall a properly sized and precharged expansion tank on the domestic hot water system, or install a thermal expansion relief valve piped to a safe drain. Closed systems with backflow prevention require this under plumbing code.
Restore downstream pressure regulation. Reinstall a pressure-reducing valve downstream of the RPZ set to protect fixtures and maintain the booster setpoint. Without it, the booster and the RPZ see uncontrolled downstream pressure.
Verify the RPZ after correction. Perform a full field test (air in water, check valve differential, relief valve opening point) to confirm the assembly was not damaged by repeated relief discharge. Document results on the test report.
Confirm no cross-connection was created by the renovation - inspect for unprotected hose bibs, chemical dispensers, and irrigation tie-ins added during construction.
Notify the purveyor and correct the record if the assembly was tagged as failed. The device was not defective; the installation was.
3. If the property manager insists on replacing the RPZ anyway, what should the inspector document and why?
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Document the actual root cause in writing: downstream pressure of 118 psi, absence of thermal expansion control, and absence of a downstream PRV. Note that the assembly passed or was not proven failed by test. This protects the purveyor and the tester from a false 'defective assembly' narrative and prevents the same failure from recurring on the new device.
Also document that replacement does not cure the expansion problem - a new RPZ will discharge identically until expansion control is restored. Record the corrective actions taken, the date, and the retest results. If the property refuses correction, the purveyor may pursue enforcement because a continuous discharge indicates an unprotected condition and possible hazard.