Top-Floor Pressure Failure and Pump Short-Cycling in a Hotel
A five-storey, 60-room hotel is fed from a 2-inch city service at 45 psi. A 1.5 kW booster pump draws from a 1,500 L pneumatic tank and discharges into a 3-inch riser. At the top-floor shower, static pressure is 14 psi; when six showers and two flushometer valves operate, pressure drops to 5 psi and the pump starts and stops every 6 to 8 seconds. The ground-floor PRV is set at 70 psi. The owner reports water hammer at the laundry and a burned pump contactor. The building has four floors of guest rooms above the ground floor, each 3.2 m floor-to-floor, and the highest fixture is 1.2 m above the fifth-floor slab. The pump curve shows shutoff head of 55 psi and rated flow of 80 gpm at 40 psi. No expansion tank is installed. The maintenance staff proposes raising the PRV setting to 85 psi.
1. Calculate the elevation pressure loss to the top fixture and determine whether 14 psi static is consistent. Does static pressure prove the system is adequate?
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Elevation difference from the ground-floor pump discharge to the highest fixture is 4 x 3.2 m = 12.8 m, plus 1.2 m = 14.0 m. Convert to feet: 14.0 / 0.3048 = 45.9 ft. At 0.433 psi/ft, the elevation loss is 19.9 psi. If the ground-floor pressure were 45 psi, static pressure at the top would be about 25 psi; the observed 14 psi means another 11 psi is being lost through the PRV, pump setting, or piping. Static pressure is not the design test. When six showers and two flushometer valves open, friction and simultaneous demand add perhaps 10 to 15 psi of loss. A residual 5 psi is below the code minimum and far below the 15 psi needed for flushometer valves, so the system fails. The reviewer should require a hydraulic calculation for peak demand, not a PRV adjustment.
2. Diagnose the short cycling and water hammer, and specify the corrections.
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The pump starts every 6 to 8 seconds because the pneumatic tank has lost its air charge or is waterlogged, so there is almost no drawdown volume. Add a properly sized bladder expansion tank, check the air charge, and set the pressure switch with a 20 psi differential, for example 40/60 psi. The 80 gpm at 40 psi pump curve may be marginal for six showers plus two flushometers; calculate peak demand and select a pump that stays on its curve. Water hammer at the laundry comes from quick-closing solenoid valves and unanchored pipe; install water hammer arrestors at the last fixture on each branch, anchor the pipe, and check the PRV. Do not raise the PRV to 85 psi; that exceeds the typical 80 psi maximum at fixtures and increases leak and fitting stress.
3. Should the PRV be raised to 85 psi? What are the risks and the correct sequence of fixes?
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No. Raising the PRV setting only raises static pressure; it does not add drawdown or pump capacity and may exceed the 80 psi fixture limit. It will worsen water hammer and may damage the PRV and fixtures. The correct sequence is: restore tank drawdown and air charge; verify pump capacity against peak demand; check the riser and fixture-supply sizes for excessive friction; add a booster or a larger riser if residual pressure at the top fixture cannot be maintained; and set the PRV to protect the lower floors, not to compensate for an undersized system. The plumber should document test readings at static and flowing conditions and retain the pump curve and hydraulic calculation in the project record.