Functional Test Fails on Economizer Changeover
A rooftop packaged unit serving a 6,000 ft² open office area is undergoing functional performance testing. The unit has a dry-bulb economizer with a differential enthalpy changeover and a modulating outdoor air damper. The test procedure calls for simulating outdoor air conditions and verifying that the economizer enables when outdoor enthalpy is below return enthalpy and disables when above. During the test, the economizer enables at the correct setpoint, but the outdoor air damper strokes from 0% to 100% in about 8 seconds and the return air damper does the opposite, causing the mixed air temperature to swing 18 °F. The supply fan VFD also hunts between 40% and 65% speed. The controls contractor says the damper actuator is 'just fast' and that the BAS trend will smooth out. The owner's design intent calls for stable space temperature within ±2 °F.
1. Is this a pass or fail, and how should the Cx provider document and resolve it?
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This is a fail of the functional test as written, for two reasons: the acceptance criterion (stable space temperature within ±2 °F) is not met, and the observed behavior (damper stroke in 8 seconds, 18 °F mixed air swing, VFD hunting) is a control instability, not a cosmetic issue.
The provider should:
- Record the failure with trend data: damper position, mixed air temperature, fan speed, and space temperature over the test period.
- State the acceptance criterion from the OPR/BOD and the test procedure, and show the measured deviation.
- Reject the 'trend will smooth out' explanation - a trend documents the instability; it does not cure it. The provider should require the controls contractor to demonstrate stable control, not merely log it.
- Identify likely causes for the contractor to investigate: actuator stroke time and range mismatch, missing or incorrect PID tuning, damper linkage slip, mixed air sensor location, or minimum-position logic conflicts.
- Require a retest after corrective action, with the same trend points and a defined stabilization period (e.g., 30 minutes at steady load).
- Track as an open issue in the issues log with responsible party and due date, and carry it into the final report if unresolved.
2. The contractor proposes to change the acceptance criterion to 'damper modulates proportionally' instead of the ±2 °F space temperature requirement. How should the provider respond?
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The provider should decline to weaken the criterion unilaterally. Acceptance criteria flow from the OPR and BOD, and the ±2 °F requirement is the owner's stated need. The provider should:
- Explain the hierarchy: the functional test verifies the design intent, not the contractor's convenience. A proportional damper that still produces an 18 °F swing does not satisfy the owner's requirement.
- Offer a legitimate path: if the contractor believes ±2 °F is unachievable with the installed equipment, the contractor should raise it to the design team and owner as a design issue, with data. The owner may then accept a revised criterion in writing - but that is an owner decision, not a contractor or Cx provider decision.
- Document the request and the disposition so the final report reflects how the criterion was resolved.
- Keep the test open until either the original criterion is met or a written owner-approved change is in place.