Economizer and Demand-Control Ventilation Fault in a Packaged Rooftop
A 20-ton R-410A packaged rooftop unit with two cooling stages, an economizer, and demand-control ventilation serves a 4,000 ft² office. Occupants complain of stale air and headaches in the afternoon. Outdoor air is 68°F dry bulb and 55 percent relative humidity; return air is 74°F dry bulb and 62°F wet bulb. The mixed-air temperature is 72°F. The economizer actuator has 24 V at the motor, but its position feedback is 10 percent. The economizer minimum position potentiometer is set at 20 percent. The CO2 sensor in the return duct reads 1,150 ppm; outdoor CO2 is 420 ppm. The enthalpy control indicates outdoor air is suitable for free cooling. Both compressors are off, and the supply fan is running. The building automation system is commanding the economizer open, but the damper linkage at the actuator crank arm is visibly disconnected. The filter is clean, and the evaporator coil is clean. The facility manager wants to disable the economizer to stop complaints.
1. Interpret the mixed-air temperature and CO2 readings. Why is disabling the economizer an inappropriate response to the complaints?
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With outdoor air at 68°F and return air at 74°F, a mixed-air temperature of 72°F indicates that the outdoor-air damper is only slightly open, roughly in the 10 percent to 20 percent range. That is consistent with the actuator feedback of 10 percent and with the disconnected linkage. The CO2 reading of 1,150 ppm in the return duct, compared with about 420 ppm outdoors, indicates inadequate ventilation for the occupant load. Demand-control ventilation should be increasing outdoor airflow above the minimum when CO2 rises, but the damper cannot respond because the linkage is disconnected.
Disabling the economizer would not fix the stale-air complaint and would make it worse by eliminating both free cooling and controlled ventilation. The unit is already not using the economizer effectively. The correct response is to repair and recalibrate the economizer so it can open for free cooling and so the minimum and demand-controlled ventilation positions are restored.
2. Diagnose the economizer problem and list the checks needed to restore proper operation.
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The 24 V at the actuator motor shows that the control signal is present, but the 10 percent feedback and the visibly disconnected crank-arm linkage show that the damper is not following the command. The actuator itself may still be functional, or it may have slipped internally; the linkage must be reconnected and the actuator stroke verified. The technician should check the actuator rotation direction, the crank-arm set screw, the damper blade shaft, and the minimum-position adjustment. Command the economizer fully open and fully closed from the building automation system or test terminals and confirm that the feedback tracks the commanded position. Verify that the outdoor-air damper, return-air damper, and exhaust damper move together as designed. Check the enthalpy control and changeover setting, then confirm that the minimum position is at least the design ventilation setting. After repair, calibrate the actuator and adjust the minimum position to the design value.
3. How should the technician verify that ventilation and free cooling are actually working after the repair, and what should be documented?
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Verification should include commanding the economizer to minimum, full open, and closed positions while observing actuator feedback and damper blade movement. With outdoor air at 68°F and return air at 74°F, a fully open economizer should produce a mixed-air temperature much closer to 68°F than the previous 72°F, and the compressors should remain off if the space load can be met. The technician should measure CO2 over time; with proper ventilation, the return CO2 should drop toward the outdoor level or toward the demand-control setpoint. The minimum position should be checked against the design ventilation requirement, and the demand-control sequence should be verified by raising CO2 above setpoint and confirming that the damper opens beyond minimum. Documentation should include actuator feedback, mixed-air temperature, CO2 before and after, minimum-position setting, and any linkage or calibration repairs. The key judgment is to restore ventilation and economizer operation rather than disabling a system that is failing because of a mechanical linkage fault.