What does open circuit capacity refer to in relation to auxiliary fans?

• A. The maximum airflow when unloaded.
• B. The maximum electrical power consumption.
• C. The physical size of the fan.
• D. The noise level produced.

Answer: (A)

Open circuit capacity is the maximum amount of air a fan can move when there is no resistance in the system—essentially the free-air or unloaded flow. In other words, it’s the airflow the auxiliary fan would deliver with zero backpressure (no ducting losses, no obstacles). This value is what sits at the top of the fan’s performance curve, and the actual operating airflow in a ventilation system is lower once you add ductwork, seals, and other resistances that create pressure the fan has to overcome. As resistance increases, the operating point shifts to lower flow and higher pressure along the curve.

It’s not about electrical power consumption, physical size, or noise, though those factors influence how the fan performs.