Improved Air Volume Control Logic for VAV Systems

In a VAV (variable air volume) system, the set point of the static pressure (usually measured at 213 downstream of the main supply air duct) is maintained by modulating the fan speed or inlet guide vane position. The set point of the static pressure is often set at a constant value, which is adequate to provide design flow to each room. This method consumes more fan power than necessary under partial load conditions. The fan speed or inlet guide vane can also be controlled based on the maximum damper position of the terminal boxes. This method is called air volume control logic. Under this control logic, terminal box airflow requirements are met with reduced static pressure. Fan power consumption is minimized. However, the actual system performance also depends on other factors, such as inaccurate temperature or flow sensors, faulty control valves, unbalanced loads, etc. This paper discusses the existing air volume control logic, its reliability and an improved air volume control logic