T M H L S B M B The fan powered box illustrated has an air flow controller that senses the velocity pressure exerted by the primary airflow. The airflow controller sets the maximum and minimum amount of air allowed through the fan powered box on cooling mode. The thermostat is allowed to vary the primary airflow between these two limits, modulate the heating valve and cycle the fan. On a drop in temperature, the thermostat reduces the cooling primary air from maximum to minimum airflow. If the temperature drops further the control valve will open to the heating coil and then closes the pressure switch, increasing the airflow through the coil on heating mode. A backdraft damper prevents primary air exiting to the ceiling space. The primary supply fan may be shut down at night and the primary damper in the fan powered boxes close. The fan powered boxes may be cycled at the night set back temperature. The main duct pressure is controlled based on input from a pressure sensor located normally twothirds 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 the required duct static pressure. On some systems the supply fan’s inlet vanes are controlled based on supply duct pressure and the return fan’s inlet vanes are controlled from building pressure. More advanced systems control the supply duct pressure just high enough to satisfy the VAV terminal with the greatest cooling requirement. Most VAV 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 cannot shine on all sides of the building at one time; therefore, full cooling should not be required everywhere, at any one time. When the air volume required exceeds the maximum volume capability of the supply fan, the duct static will drop below the design value. Some factors causing this are supply duct leaks, VAV boxes allowing more than their original design, more VAV boxes added to the system, cooling load exceeding original design or the supply fan providing less volume than its original design. 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 also vary. This fact is rarely considered in setting the minimum ventilation logic. PARALLEL FAN POWERED BOX 7.56 RETURN FAN SUPPLY FAN INLET VANES OR VFD PRESSURE SENSOR SUPPLY DUCT MIXED AIR NC NC NO INLET VANES OR VFD RETUEN AIR OUTSIDE AIR EXHAUST AIR TEMPERATURE SENSOR TEMPERATURE SENSOR TEMPERATURE SENSOR FAN POWERWD BOX FILTER FAN BACKDRAFT DAMPER PE ROOM AIR CONTROL VALVE THERMOSTAT AIR FLOW CONTROLLER COOLING COIL HEATING COIL
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