Overtime Chiller Sequencing on a Tight Handover Date
A 14-story office building has three 600-ton water-cooled centrifugal chillers, variable primary flow, and a design condition of 95°F dry bulb. The Cx agent is asked to run functional performance testing (FPT) two weeks before substantial completion because the owner wants to occupy floors 1-6. The mechanical contractor reports that only two chillers have operating BAS sequence logic loaded; the third is installed and wired but its factory start-up report is missing. The controls subcontractor says the staging logic was 'bench tested' and offers a trend log from a 4-hour run at 60°F outdoor air. The owner's representative suggests signing off on the two working chillers and 'finishing the third later' so the occupancy date holds. The schedule shows the third start-up report will arrive in nine days.
1. What should the Cx agent do about the proposed partial sign-off, and what does the commissioning plan require here?
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The Cx agent should decline to treat the two-chiller demonstration as acceptance of the chiller plant sequence. The FPT procedure is written against the design intent, which is full-load and part-load staging across three machines; a 60°F-ambient, 4-hour trend at light load exercises neither the staging thresholds nor the lead/lag rotation. Bench testing verifies logic in isolation, not field wiring, sensor calibration, or the actual sequence under real load.
The correct path is to hold the plant sequence FPT open as a documented incomplete item, not a passed test. The Cx plan should already define prerequisites for each test: equipment started by the manufacturer, controls points verified, sensors calibrated, and safeties proven. The missing factory start-up report on chiller 3 is a prerequisite failure, so the test cannot be scheduled as a pass. The agent can offer a conditional, interim demonstration of the two available machines to support partial occupancy, but it must be labeled a partial test with the full sequence still pending. The owner can accept the building with an open commissioning issue and a defined completion date, which is a business decision the owner is entitled to make, but the Cx agent should not sign a test report that says the sequence passed. The nine-day gap should be used to complete point-to-point verification and sensor calibration so the full test can run immediately when the report arrives.
2. How should the Cx agent respond to the controls subcontractor's trend log as evidence of performance?
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The trend log is useful but insufficient as acceptance evidence. The agent should evaluate it for what it actually proves: that the loaded logic executed without alarms at light load. It does not prove staging at the design changeover points, does not prove lead/lag rotation or failure response, and does not prove that the chiller current-limit and minimum-flow safeties interact correctly with the sequence.
The agent should ask for the trend interval (5-minute data cannot resolve a 10-minute staging delay), the sensor calibration records behind the logged values, and the outdoor-air conditions at the time. Then the agent should require the full test at a load that can be produced, which may mean a load bank, temporary boilers, or waiting for warmer weather. If the season prevents a design-condition test, the plan's seasonal-testing provision applies: run what can be run now and schedule a deferred seasonal test with a named responsible party and date, recorded as an open item in the issues log. The agent should not let a favorable ambient run substitute for the deferred test; it should be documented as a partial verification only.