Economizer Damper Hunting on a Packaged Rooftop Unit
A 40-ton packaged RTU with factory DDC serves a single-zone assembly hall. The TAB firm is called back two weeks after final balancing. The client reports the space swings between 68°F and 77°F each afternoon. Trend logs show the economizer actuator driving from 0% to 100% and back roughly every 6 minutes whenever outdoor air is between 55°F and 65°F. Mixed-air temperature at the sensor oscillates about 8°F around setpoint. The TAB report shows outside air balanced to 4,800 cfm at design, return fan tracking, and static pressure at the mixed-air plenum holding steady. The economizer changeover setpoint is a fixed 60°F dry bulb with a 2°F differential. The engineer asks the TAB technician whether the balancing is at fault and what should be done before touching any damper linkage.
1. Diagnose the most likely root cause of the oscillation and state whether the TAB balancing is implicated.
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The symptom pattern - fixed dry-bulb changeover with a 2°F differential, hunting every ~6 minutes, mixed-air temperature swinging 8°F - points to a controls logic and sensor-placement problem, not a balancing problem.
Reasoning:
- The OA is balanced to design and the mixed-air plenum static is stable, so the dampers are moving air as commanded. The TAB work is not implicated.
- A fixed 60°F changeover with only a 2°F differential sits right at the edge of the afternoon OA temperature band (55–65°F). When OA crosses the changeover, the economizer enables; once enabled, cold OA over-cools the mixed air, the controller drives the OA damper toward minimum, mixed air warms, OA crosses back, and the cycle repeats. That is classic integral windup with a too-narrow deadband.
- The mixed-air sensor is likely located close to the OA/RA mixing point, so it sees stratified air and responds to the damper position rather than the true mixed condition - amplifying the swing.
Conclusion: root cause is economizer control tuning (changeover differential and/or PID) plus mixed-air sensor location/stratification. TAB balancing is not at fault.
2. List, in order, the verification steps the TAB technician should perform and document before recommending a controls change.
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Do these in order and record each result:
- Verify actual OA/RA/MA airflow at the current damper positions using a pitot traverse or flow station - confirm the OA is still at 4,800 cfm and return tracking is intact. Rule out a slipped linkage.
- Verify mixed-air sensor location and stratification - measure temperature at multiple points across the MA duct cross-section (a grid traverse) and compare to the controller's sensed value. A spread >3°F confirms stratification.
- Verify the OA temperature sensor - compare to a calibrated reference at the intake; a biased sensor changes when the economizer thinks it should switch.
- Review the trend data to confirm the enable/disable frequency and correlate with OA temperature.
- Document findings in a written field report with the traverse data, sensor comparison, and trend evidence, and submit to the engineer of record.
Only after this evidence is assembled should the TAB firm recommend a controls fix (widen the changeover differential, add a mixed-air low-limit, relocate the MA sensor downstream of a mixing baffle). The TAB firm should not unilaterally reprogram the DDC.