Air duct static pressure is usually a control variable maintained by the supply fan for variable air volume (VAV) systems. Typically the air static pressure is set at a constant set point based on design conditions. However, under partial load conditions, terminal box dampers have to be closed more since required airflow is less than the design airflow which directly results in significantly smaller pressure loss. Thus no matter the terminal boxes are pressure dependent or pressure independent, the static pressure set point should be reset to a lower level to reduce the fan energy, the noise in the terminal boxes, and the terminal boxes malfunction. With the static pressure reset, the room condition can also be maintained better at lower fan energy consumption by minimizing simultaneous cooling and heating. This paper proposes to control the static pressure in dual-duct variable air volume systems by using supply fan variable frequency drives (VFDs) and installing hot air dampers in the hot deck. The static pressure set point is reset based on fan airflow measured by fan airflow station (FAS). With regard to affecting factors of space load, availability of terminal box damper position and space cooling demand, this integrated method has advantages over the existing measures such as fixed static pressure, static pressure reset by outside air temperature, static pressure reset by VAV box damper position and static pressure reset by cooling loop output. According to both the model analysis and case building study, it turns out that this method can significantly save fan power.

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