Ventilation Complaint in a Sealed Meeting Room
A 40 m2 meeting room with a 2.7 m ceiling has a variable-air-volume terminal serving four occupants normally. It has no operable windows and relies on a central air-handling unit for outdoor air. During a full-day workshop with 14 people, attendees report headaches and stuffiness by mid-afternoon. The CO2 reading climbs from 500 ppm at 09:00 to 2,100 ppm by 15:00 and falls only slowly after the room empties. The VAV box is at its minimum airflow position, and the central unit's outdoor-air damper is at a fixed 20 percent open. The building manager proposes fitting a CO2-controlled damper on the room terminal.
1. Estimate whether the outdoor-air provision is likely to meet the design intent for 14 occupants, and explain the CO2 trend.
Read the worked answer
The room volume is 40 x 2.7 = 108 m3. At 14 occupants that is under 8 m3 per person, which is a small volume for a full-day meeting, so CO2 will build quickly if outdoor air is short. The terminal is at minimum airflow, and the central unit's outdoor damper is fixed at 20 percent, so the outdoor-air fraction delivered to the room is a fixed share of whatever air passes through the terminal, not a quantity set for 14 people. When the terminal throttles to minimum, the absolute outdoor-air volume falls with it, exactly when occupancy is highest.
The CO2 curve confirms this. A rise from 500 to 2,100 ppm over six hours, with a slow decay afterward, is consistent with a ventilation rate well below what 14 people need. The slow fall after the room empties reflects the room's air change rate being low even with no occupants. The stuffiness and headaches are consistent with the CO2 level and with other occupant-generated pollutants that accumulate alongside it.
The design intent should be checked: the outdoor-air rate should be set from the number of occupants and the room's use, not left as a fixed damper position.
2. Evaluate the proposed CO2-controlled damper on the room terminal and state a better sequence of actions.
Read the worked answer
A CO2-controlled damper on the room terminal addresses the symptom but not the cause. Demand-controlled ventilation is useful when a space's occupancy varies and the central plant can deliver more outdoor air on demand. Here the central unit's outdoor damper is fixed at 20 percent, so even a wide-open room damper can only draw a larger share of a fixed outdoor-air fraction. The room would get more total air, but the outdoor-air quantity may still be inadequate, and the terminal's minimum flow may not allow enough air through at all.
The better sequence is: first, measure or calculate the required outdoor-air rate for 14 occupants and compare it with what the system delivers at minimum terminal flow. Second, reset the central outdoor-air damper so the air-handling unit delivers the required outdoor-air fraction at all terminal positions, or link the damper to a CO2 or occupancy signal at the unit. Third, check the terminal's minimum and maximum flows against the room's design occupancy and adjust if needed. Fourth, if the room will regularly host large meetings, consider a dedicated outdoor-air supply or a CO2 sensor that raises both the terminal flow and the central outdoor-air damper together.
Verify by re-measuring CO2 over a comparable full-day meeting and confirming it stays within the design target.