Which statement correctly describes a parallel fan in a VAV box?

• A. Parallel fan runs at full speed.
• B. Parallel fan bypasses space air.
• C. Parallel fan increases outside air delivery.
• D. Parallel fan operates only at reduced primary flow, reducing overall fan operation.

Answer: (D)

The parallel fan is used to supplement the main (primary) air flow in a VAV box and is only energized when the primary flow is reduced to meet the space load. This means the system doesn’t run the parallel fan at full speed all the time; it turns on as needed to maintain the required supply while the primary flow is decreased, which lowers overall fan energy use.

That’s why the statement describing the parallel fan as operating only when the primary flow is reduced, thereby reducing overall fan operation, is correct. The other ideas don’t align with how a parallel fan is intended to work: it doesn’t continuously run at full speed, it doesn’t bypass space air, and it doesn’t specifically increase outside air delivery.