AHU Economizer: Mixed-Air Temperature Does Not Match Command
AHU-3 serves a 20,000-cfm office zone. It has linked outdoor-air, return-air, and relief dampers, a dry-bulb economizer high limit of 65°F, and a mixed-air low limit of 45°F. Design mixed air is 55°F at 30% outdoor air. On a spring day with 56°F outdoor air, 76°F return air, and the economizer command at 100% outdoor air, the BAS mixed-air sensor reads 71°F. The outdoor-air damper actuator is commanded to 10 V and the actuator shaft rotates, but the damper blade position indicator is at about 15% open. The unit is at 80% fan speed, supply air is 58°F, and the space is 1.5°F above setpoint. The controls technician wants to recalibrate the mixed-air sensor and move on.
1. Use mass balance to test the sensor-error explanation and determine the probable cause.
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Mass balance: at 100% outdoor air, mixed air should be 56°F; at 15% outdoor air, mixed air = 0.15 x 56 + 0.85 x 76 = 73°F; at 30%, mixed air = 70°F. The measured 71°F is consistent with roughly 25-30% outdoor air, not 100%, and not with a sensor error alone. A 15°F error in a mixed-air sensor is implausible. Because the actuator shaft rotates but the blade indicator shows 15%, the likely cause is a slipping linkage or a damper blade separated from its shaft. The command and actuator are not being transmitted to the damper. Recalibrating the sensor would mask the real failure and leave the economizer ineffective.
2. Design a functional performance test for the economizer, including damper stroke, leakage, and failure modes.
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With the unit at stable full airflow, command the outdoor-air damper to 0, 25, 50, 75, and 100%. At each step record command, actuator position, blade position, outdoor, return, and mixed-air temperatures, relief damper position, fan speed, and building pressure. Verify the high-limit lockout by simulating an outdoor-air temperature above 65°F and confirming the damper closes. Verify the 45°F mixed-air low limit by simulating a low mixed-air signal and confirming the damper modulates closed. Verify spring return on power loss and on fan stop. Measure closed-damper leakage by sealing the outdoor intake temporarily or by comparing mixed-air temperature with the damper commanded closed. Observe the linkage under load, not just at the actuator.
3. State the acceptance criteria and documentation required before the economizer is accepted.
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Acceptance criteria: blade position within ±5% of command at each test point; measured mixed air within 2°F of the mass-balance prediction; outdoor-air damper closes on high limit, fan stop, and freeze protection; no coil freeze risk; and leakage at the closed position is less than 5% of design outdoor air. Document time-stamped trends, actuator and blade positions, temperature readings, and the simulated failure tests. Require repair of the linkage or damper and a complete retest, not a sensor calibration. The test form should be signed by the controls contractor and the commissioning provider.