What is the function of a series fan in a VAV box?

• A. Only cooling air passes through the fan when space cooling is needed.
• B. The fan circulates air around the fan housing without affecting primary air.
• C. All air to space flows through the fan, modulating primary air from maximum cooling to minimum.
• D. Outside air is increased.

Answer: (C)

In a series fan VAV box, the fan sits in the main airflow path so every cubic foot of supply air to the space goes through the fan. That arrangement lets the system control the space by using the damper to set how much primary air from the AHU can pass into the box, while the fan speed provides the pressure to push that air through the damper and into the space. As cooling demand changes, you can move the damper from wide open (maximum cooling) to nearly closed (minimum cooling), and the fan speed adjusts to maintain the required flow. The fan’s job isn’t simply circulating air inside the box, nor does it inherently increase outdoor air—OA is controlled by the system’s OA/Economizer controls, not the series fan.