What is the benefit of using a parallel fan in a VAV box?

• A. It provides no benefits compared to a series box.
• B. It runs at full speed regardless of demand.
• C. It bypasses the heating coil entirely.
• D. It operates only at reduced primary flow, providing similar benefits to a series box but with less fan operation.

Answer: (D)

The main idea is that a parallel fan-powered VAV box adds extra supply air without forcing the entire airflow through the coil, and it does this in a way that stays efficient at partial loads. In this setup, the primary air damper can reduce the central flow, while the local fan provides the additional volume the space needs to meet its load. Because the primary flow is reduced and the fan only runs as needed (often at a lower speed), you get energy savings compared with a high-primary-flow arrangement, while still achieving the same zone comfort benefits that a series configuration can provide. The parallel fan does not run at full speed all the time, and it does not bypass the heating/cooling coil entirely—the coil still conditions the primary air, while the fan supplements total supply.