VAV Ventilation Rate and Demand-Controlled Ventilation Acceptance
An 18,000-ft², 120-person open office has a VAV system with 24 boxes. Design outdoor air is calculated per ASHRAE 62.1-2019 Ventilation Rate Procedure: Rp = 5 cfm/person, Ra = 0.06 cfm/ft², system ventilation efficiency Ev = 0.8. The commissioning provider measures 2,300 cfm at the outdoor-air intake with the AHU at design airflow and all boxes at maximum. The BAS reports the outdoor-air damper at 100% and the return damper at 20%. The demand-controlled ventilation sequence uses a CO2 setpoint of 900 ppm, but CO2 sensors read 430 ppm in a fully occupied conference room. The owner wants to know if the system complies and whether the DCV can be accepted.
1. Calculate the required design outdoor airflow and compare it with the measured value.
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Vbz = 120 x 5 + 0.06 x 18,000 = 600 + 1,080 = 1,680 cfm. Vot = Vbz / Ev = 1,680 / 0.8 = 2,100 cfm. The measured 2,300 cfm is 200 cfm above the calculated design, about 9.5%. On its face it passes. However, a single duct traverse often has an uncertainty of plus or minus 10%, so the lower bound is near 2,070 cfm, which is too close to 2,100 cfm to accept without a calibrated flow station or repeated traverse. The result must also be corrected to design airflow conditions and fan speed if the test was not at design.
2. Determine whether the DCV can be accepted based on the CO2 readings and explain the additional ventilation test needed.
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Do not accept DCV on the basis of the CO2 readings. A 430-ppm reading in a fully occupied conference room is at outdoor-air background and is not credible; either the sensor has failed, the reading is not from that space, or the DCV sequence is not enabled. The 900-ppm setpoint is not itself proof of compliance. The measured 2,300 cfm at design airflow is also not the critical condition: VAV systems reduce the outdoor-air fraction at minimum box flow. The test must verify outdoor air at both design and minimum airflow, with DCV enabled and disabled, and must confirm that the critical zone still receives its required breathing-zone outdoor air.
3. Recommend actions and acceptance criteria before the owner accepts the ventilation system.
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Calibrate or replace the CO2 sensors using a known gas, verify sensor location and trending, and prove that the DCV sequence actually drives the outdoor-air damper. Verify that the AHU maintains at least the calculated minimum outdoor air at minimum box flow. If it cannot, add a minimum outdoor-air flow station or revise box minimums. Retest with a calibrated orifice or flow station and require measured outdoor air to be at least 2,100 cfm plus measurement tolerance at design, and at least the minimum required rate at minimum airflow. Acceptance also requires documented CO2 response to occupancy and a sequence that fails safe to the minimum outdoor-air position.