In a VAV system, how does outside air flow behave if there is no control response to decouple it from supply flow?

• A. Outside air flow will remain constant regardless of supply.
• B. Outside air flow increases with humidity.
• C. Outside air flow will vary with supply flow without control response.
• D. Outside air flow is unaffected by supply flow.

Answer: (C)

In a VAV system, outdoor air is introduced into the mix with the supply air. If there’s no independent control to decouple outdoor air from the supply path, the outdoor air flow follows the same flow changes that the supply air experiences. That means outdoor air flow is effectively proportional to the supply flow: when the supply volume increases to meet zone loads, more outdoor air is drawn in; when the supply volume drops, outdoor air flow drops too. Without decoupling, the two share the same pressure and duct conditions, so outdoor air cannot stay constant or independent of the supply.