Hydronic Loop That Won't Balance - Pump Curve vs. System Curve
A new 12-story building has a primary-secondary chilled water system. The secondary loop serves 30 fan-coil units through a reverse-return header and a variable-speed secondary pump. The TAB firm is balancing the secondary loop and finds that the farthest FCU on the top floor receives only 60% of design flow even with its balancing valve wide open. The nearest FCU receives 140% of design with its valve nearly closed. The secondary pump is running at 100% speed and the differential pressure at the pump is 10 psi below the design value. The pump curve shows design at 800 gpm and 60 ft head. The system has a 3-way valve at the nearest FCU that is stuck in the bypass position. The engineer wants the TAB firm to "just open the far valve more."
1. Explain why opening the far valve more will not solve the problem, using system-curve reasoning.
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Opening the far valve more lowers that branch's resistance, but the problem is not branch resistance - it is flow distribution driven by a stuck 3-way valve and an operating point off the design curve.
Reasoning:
- The stuck 3-way valve at the near FCU is bypassing flow back to the return, effectively creating a low-resistance short circuit near the pump. That pulls the system curve flatter, so the pump operates at higher flow and lower head than design (consistent with the 10 psi low differential).
- With a flatter system curve, the pump's available head at the far branch is reduced, so the far FCU starves - no amount of opening its valve recovers head that the pump is not producing.
- The near FCU at 140% confirms the short-circuit is stealing flow.
Opening the far valve further cannot fix a head deficit at the far end; it only marginally changes that branch's resistance while the real fault (the bypass) remains. The fix is to repair the stuck 3-way valve, then re-trim the loop.
2. Outline the correct balancing sequence and the pump/differential-pressure checks to perform.
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Sequence:
- Fix the fault first. Have the 3-way valve repaired/replaced and confirmed in the correct position. Re-establish design conditions before any balancing.
- Verify pump performance. Measure pump suction and discharge pressure with calibrated gauges, compute actual head, and read flow (ultrasonic or flow meter). Plot the actual operating point against the manufacturer's curve. Confirm speed and impeller trim.
- Check the differential-pressure sensor location and setpoint for the variable-speed control; confirm it reflects the critical (farthest) branch, not the near branch.
- Balance proportionally. With all valves open, measure each FCU, compute the ratio of measured to design, and set the valve on the lowest-ratio unit as the reference. Adjust the others to match that ratio, working from the reference outward. Do not fully close any valve.
- Re-check total flow against design and adjust the secondary pump speed or trim if the total is off.
- Document pump curve point, valve positions, and final flows. Verify the far FCU now meets design within tolerance before sign-off.