Conference Room CO2 and Minimum Outdoor Air
A 600 ft² conference room with a 9 ft ceiling is served by a VAV terminal with a design airflow of 900 cfm and a minimum airflow of 270 cfm. The room is designed for 20 occupants. At 2:00 p.m., with 20 people in the room, the building automation system shows a CO2 concentration of 1,350 ppm; outdoor CO2 is 420 ppm. The VAV box is at minimum airflow, and a balometer reading at the outdoor-air intake hood is 120 cfm. The outdoor-air damper is commanded to its minimum position, but the linkage set screw is loose and the blade is visibly at a lower angle. The design outdoor-air intake at 900 cfm is 180 cfm. The system uses single-duct VAV with a flow-measuring station at the air handler.
1. Calculate the minimum outdoor-air rate required for this room under the ASHRAE 62.1 Ventilation Rate Procedure and compare it with the measured rate.
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Using the breathing-zone ventilation rate for a conference room, the people component is 5 cfm per person and the area component is 0.06 cfm per ft². For 20 people, the people component is 20 times 5, or 100 cfm. For 600 ft², the area component is 600 times 0.06, or 36 cfm. The total breathing-zone outdoor airflow is 136 cfm, assuming a zone air distribution effectiveness of 1.0. No zone-level correction is needed for a single-zone calculation here. The measured outdoor-air intake is 120 cfm, which is 16 cfm below the required 136 cfm, or about 12% low. The measured rate is therefore not adequate for the design occupancy. The loose damper linkage explains why the commanded minimum position is not the actual position, so the system is delivering less outdoor air than the design intent at minimum airflow.
2. Use a CO2 mass balance to check whether the measured 120 cfm is consistent with 1,350 ppm, and state the limitations of that estimate.
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Assume a CO2 generation rate of about 0.0055 cfm per person for light office activity. For 20 people, generation is 20 times 0.0055, or 0.11 cfm. At steady state, the indoor-outdoor CO2 difference equals generation divided by outdoor airflow, multiplied by 1,000,000. With 120 cfm, the predicted difference is 0.11 divided by 120, times 1,000,000, or about 917 ppm. Adding the 420 ppm outdoor concentration gives about 1,337 ppm, which is close to the measured 1,350 ppm. The estimate supports the conclusion that outdoor airflow is near 120 cfm and not 136 cfm. Limitations include unknown actual occupancy, transient conditions, imperfect mixing, CO2 sensor calibration, and the assumed generation rate. CO2 is an indicator, not a direct airflow measurement, so the balometer reading and damper position remain the primary evidence.
3. Recommend corrective actions and describe how you would verify the fix.
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Repair or replace the damper linkage and set the minimum position so that the measured outdoor-air intake is at least 136 cfm when the VAV box is at its 270 cfm minimum. Because a fixed damper position delivers less outdoor air as supply airflow falls, verify the minimum at the minimum box airflow, not only at design airflow. If the damper cannot achieve 136 cfm at minimum airflow, consider a flow-measuring outdoor-air station with reset, or a demand-controlled ventilation sequence that modulates the damper based on CO2 and occupancy. After adjustment, measure outdoor airflow with a balometer at the intake hood, confirm the VAV box minimum with the flow station, and check that the damper actuator strokes freely and holds position. Verify the CO2 sensor against a calibrated reference, and log CO2 during a full occupied period. Confirm that the room remains below the design target and that the system does not over-ventilate at design airflow.