What is the main challenge with outside air in a VAV system regarding outside air delivery?

• A. Outside air flow will remain constant regardless of supply.
• B. Outside air flow will vary with supply flow without control response.
• C. Outside air flow is independent of supply flow.
• D. Outside air flow always increases with higher supply.

Answer: (B)

In a VAV system, outdoor air delivery is not automatically fixed or independent of how much air the system is delivering to the space. The outdoor air damper and the mix with return air are driven by the same control logic that modulates the supply volume. If there’s no active control responding to occupancy, CO2, outdoor conditions, or economizer/DCV logic, the amount of outdoor air that actually enters the space will change as the total supply flow changes. When the VAV box reduces or increases its supply to meet load, the absolute outdoor air flow tends to track that change, so ventilation rates can wander away from the required target. That’s why having a control response to adjust outdoor air in concert with supply and occupancy is essential.