VAV (VARIABLE AIR VOLUME) SYSTEM This type of system allows each area’s thermostat to vary the amount of cold air entering the occupied space, addressing the varying cooling loads. The VAV box illustrated has an air flow controller that senses the velocity pressure exerted by the airflow. The airflow controller sets the maximum and minimum amount of air allowed through the VAV box. The thermostat is allowed to vary the airflow between these two limits. This arrangement is referred to as pressure independent, as the airflow controller tends to correct for varying upstream static pressure values. The airflow controllers are proportional with no integral reset; therefore, significant shift in control point is experienced if the static pressure variation is excessive. The main duct pressure is controlled based on input from a pressure sensor located normally two-thirds of the way down the duct, based on air volume. The controller varies the position of the inlet vanes on the supply and return fans or controls variable frequency drives for these fans, maintaining constant duct static pressure. Most systems have a diversity factor. This factor is the percentage difference between the total maximum volume of the VAV boxes and the maximum CFM of the supply fan. This is based on the fact that the sun can not shine on all sides of the building at one time; therefore, full cooling will not be required every where, at any one time. The minimum ventilation requirement of a building is usually a fixed CFM value and the setting is normally a percentage of the supply fan’s volume. The fact that the supply fan volume varies, requires that the relative percentage of air for minimum ventilation must vary also. This fact is rarely considered in setting the minimum ventilation logic. VAV SYSTEM PRESSURE INDEPENDENT RETURN FAN EXHAUST NC RETURN AIR AIR RETURN INLET VANES VAV NO (OR VFD) THERMOSTAT SUPPLY FAN AIRFLOW CONTROLLER SUPPLY OUTDOOR NC MIXED AIR AIR AIR SUPPLY INLET VANES (OR VFD) 7.54 M M
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