Why is open loop fan tracking generally considered problematic in HVAC fan control?

Why is open loop fan tracking generally considered problematic in HVAC fan control?

• A. The speed-flow relation is not linear.
• B. The speed-flow relation is exactly linear.
• C. Open loop tracking is independent of fan speed.
• D. It is unaffected by outdoor air.

Answer: (A)

Open loop fan tracking is problematic because the relationship between fan speed and the actual airflow delivered by a system is not a simple, predictable line. In real ductwork, the flow you get from a given speed depends on system pressure, which changes as dampers move, outdoor air fraction varies, filters clog, and duct losses differ along the path. Because of these nonlinearities and changing operating points, a fixed speed-to-flow map used in open loop cannot reliably produce the desired airflow across all conditions. With that nonlinearity in mind, a closed-loop approach—using sensors to measure actual flow or pressure and adjust speed accordingly—compensates for the changing system resistance and external factors, producing the intended ventilation and comfort more consistently. Outdoor air affects system pressure and flow as well, so ignoring it in open loop further degrades accuracy. The other statements aren’t correct because the speed-to-flow relationship is not exactly linear, open loop tracking is not independent of fan speed, and outdoor air does influence the system’s behavior and required fan performance.

Open loop fan tracking is problematic because the relationship between fan speed and the actual airflow delivered by a system is not a simple, predictable line. In real ductwork, the flow you get from a given speed depends on system pressure, which changes as dampers move, outdoor air fraction varies, filters clog, and duct losses differ along the path. Because of these nonlinearities and changing operating points, a fixed speed-to-flow map used in open loop cannot reliably produce the desired airflow across all conditions.

With that nonlinearity in mind, a closed-loop approach—using sensors to measure actual flow or pressure and adjust speed accordingly—compensates for the changing system resistance and external factors, producing the intended ventilation and comfort more consistently. Outdoor air affects system pressure and flow as well, so ignoring it in open loop further degrades accuracy.

The other statements aren’t correct because the speed-to-flow relationship is not exactly linear, open loop tracking is not independent of fan speed, and outdoor air does influence the system’s behavior and required fan performance.