Variable Primary Chilled Water Plant: Remote DP and Low Delta-T
At a 12-story office, the new chilled water plant is variable primary flow: two 600-ton chillers, three 400-gpm pumps, design 1,200 gpm at 12°F delta-T, and a bypass valve across the plant headers. The differential pressure sensor was installed at the pump discharge header, and the BAS controls pump speed to hold 45 ft there. The design remote DP setpoint at AHU-7 is 30 ft. During a 40% load test, AHU-7, the hydraulically farthest unit, cannot maintain space temperature; its coil valve has been 100% open since 09:00. The far-end DP gauge reads 18 ft. Plant flow is 610 gpm, supply water 44°F, return 52°F. The bypass valve is 35% open, and the lead chiller is at its minimum load. The controls contractor says the pump is maintaining setpoint and the problem is the AHU coil. The owner wants the plant accepted.
1. Determine the most probable root cause and explain the hydraulic and control mechanism, including why the bypass is open and the plant delta-T is low.
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The most probable cause is the DP sensor location. With the sensor at the pump discharge header, the BAS holds 45 ft at the plant, not 30 ft at AHU-7. At part load the pump slows until the header DP reaches setpoint; the far-end DP then falls to 18 ft, so AHU-7's valve stays wide open but cannot get design flow. The bypass valve opens because the plant flow controller is trying to maintain minimum chiller flow while the coil valves are starved; only part of the 610 gpm reaches the coils, so the plant delta-T is 8°F instead of 12°F and the lead chiller appears lightly loaded. The AHU coil is a symptom, not the cause. The controls contractor's statement that the pump is maintaining setpoint is true only at the wrong point.
2. List the measurements and test steps needed to confirm the diagnosis before any modification, and state acceptance criteria.
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Before changing anything, record pump speed, pump head, header DP, remote DP, bypass position, chiller evaporator flow and delta-T, and AHU-7 valve command and position. Install a calibrated differential pressure gauge at AHU-7 and across the farthest coil. Perform a manual pump-speed step test at 50, 70, 90, and 100% and record remote DP, coil flow, and valve position. Force the bypass closed briefly and see whether remote DP rises. Confirm whether the remote DP responds to pump speed at the critical coil, not just at the header. Acceptance criteria: at design flow, remote DP is at least 30 ft, all coil valves are below 95% open at design load, plant delta-T is within 1°F of design, and the bypass is closed above minimum chiller flow. If those are not met, the sensor location and control logic are the problem.
3. Recommend the corrective action and describe how to re-test the plant.
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Relocate the DP sensor to the hydraulically remote coil, or use a remote DP transmitter with a reset schedule, and set the remote DP setpoint to 30 ft. Revise the bypass control to maintain chiller minimum flow using a flow meter rather than header DP. Add a pump-speed low limit and verify sensor range and calibration. Re-test with available load or a load bank: confirm remote DP, valve positions, delta-T, and bypass closure at 40, 70, and 100% load. Trend for a week to confirm the pump does not hunt and the bypass stays closed except when chiller minimum flow requires it. Document the before-and-after readings and obtain owner acceptance of the revised sequence.