Low Pressure at the Top Floor After a Device Swap
A six-story building had its old reduced-pressure assembly replaced with a new 2-inch RP. Static pressure at the street is 65 psi. The old device had a rated pressure loss of about 6 psi at design flow; the new one is rated at 12 psi at the same flow. Two weeks after the swap, tenants on floors 5 and 6 complain of weak flow at fixtures. The installer measured 58 psi static on the downstream side with no flow, and 44 psi residual at a measured flow of 90 gpm. The building's design demand is 110 gpm. Elevation from the meter to the sixth floor fixtures is about 60 feet. The manager wants to remove the RP to restore pressure.
1. Using the numbers given, explain why the upper floors lost pressure and whether removing the RP is a valid fix.
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Two effects combine. First, elevation: 60 feet of height costs roughly 0.433 psi per foot, or about 26 psi of static pressure, so the sixth floor sees far less pressure than the street regardless of the device. Second, the new RP adds more friction loss than the old one. At 90 gpm the measured residual is 44 psi, and the design demand is 110 gpm, which is beyond the tested point; friction loss rises roughly with the square of flow, so at 110 gpm the loss will be substantially higher than at 90 gpm. The device was likely selected without checking its pressure-loss curve against the building's actual demand and the elevation penalty. Removing the RP is not a valid fix: it eliminates required health-hazard protection to solve a sizing problem. The correct remedies are to select an assembly with a lower loss at design flow, upsize the device or piping, boost pressure with a properly protected booster system, or rebalance the distribution. The fix must preserve protection.
2. What additional measurement would you take to confirm the diagnosis before recommending a remedy?
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Measure pressure at several points simultaneously: upstream and downstream of the RP, and at a fixture on the sixth floor, while flowing a known rate. A single downstream reading cannot separate elevation loss from device loss. Recording upstream pressure during flow shows whether the street supply itself sags at demand; recording the differential across the device at a measured flow lets you compare actual loss to the manufacturer's curve. Also flow the system near the 110 gpm design demand, not just 90 gpm, because the loss curve is nonlinear and the tested point understates the problem. Confirm the fixture height by measurement rather than estimate, and check for other restrictions such as a partially closed valve or undersized downstream piping. With all readings taken at the same time and flow, the cause becomes attributable rather than assumed.