Which statement accurately describes a series fan in a VAV box?

• A. All air to space flows through the fan, modulating primary air from maximum cooling to minimum.
• B. The space air bypasses the fan when cooling is needed.
• C. The space air bypasses the fan entirely.
• D. The series fan reduces the need for outside air.

Answer: (A)

In a series fan-powered VAV box, the air going to the space must pass through the fan as part of the delivery path. The primary air damper modulates how much primary (ventilated, conditioned) air is supplied, while the fan adjusts the total flow to meet the zone’s cooling or heating load. So the whole air that reaches the space goes through the fan, and the system modulates the primary air from a high flow (maximum cooling) down to a reduced flow (minimum cooling) as needed, with the fan providing the needed boost or reduction in overall airflow.

Other descriptions would imply the space air bypasses the fan (not the case in the series arrangement) or would inaccurately claim the series fan changes outside air requirements.