What is the correct interpretation of the statement 'As the supply air cfm is reduced in a VAV system, outdoor air cfm is reduced'?

• A. Outdoor air is unaffected by supply air changes
• B. Outdoor air cfm is reduced along with supply air
• C. Outdoor air cfm increases when supply air decreases
• D. Outdoor air cfm remains constant

Answer: (B)

In a VAV system, outdoor air is brought in through a damper that feeds into the same airstream the VAV box controls. The outdoor air flow is typically tied to the amount of supply air being delivered, often expressed as a fixed percentage of the supply air. When the VAV box reduces its supply air cfm to meet a smaller cooling or heating load, the outdoor air portion that mixes with that supply also drops proportionally. So, outdoor air cfm decreases along with supply air.

It’s worth noting that many systems still must meet a minimum outdoor air requirement to satisfy ventilation standards. In those cases, the outdoor air flow won’t fall below that minimum, but the general behavior in typical control schemes is that outdoor air tracks the supply air.