IMPACT OF DEFECTIVE AIRFLOW CONTROLLER OR BLOWN DIAPHRAGM ON A PNEUMATIC VAV BOX VAV BOX IS 8” WITH AN AIRFLOW CONTROLLER RESET RANGE OF 8 PSIG TO 13 PSIG. THERMOSTAT SIGNAL BELOW 8 PSIG; THERMOSTAT SIGNAL ABOVE 13 PSIG; THEREFORE, MINIMUM AIRFLOW SETTING. THEREFORE, MAXIMUM AIRFLOW SETTING. VELOCITY PRESSURE .03” WG = 150CFM VELOCITY PRESSURE .14” WG = 340CFM BLOWN DIAPHRAGM VELOCITY PRESSURE .23” WG = 450 CFM VAV CALIBRATION CHART (8” BOX) (DAMPER WIDE OPEN) The diversity factor on a VAV system is the percentage the fan is undersized relative to the total of all the VAVs’ maximum airflow settings. (Example: If a fan can pass 100,000 CFM and the VAV’s total maximum capability is 115,000 CFM, the diversity factor is 15%.) This leaves the fan 15% undersized, but the sun cannot shine on all building faces at once, so maximum airflow should never be required and the duct static pressure setting of (example) 1” WG should be attainable. The above example of defective airflow controller or blown diaphragm demonstrates that this VAV passes 32% more system air than when working properly at maximum air flow. Once issues cause the system demand to exceed the example fan’s 100,000 CFM the duct static pressure set point of 1” WG cannot be maintained and the duct static could drop to a level we have witnessed of, for example .4” WG. The airflow controllers’ data sheets claim to be pressure independent, but as per our VAV case study this is not true if the duct static variation is large. Calibrating a building at various duct static pressures can result in a very poor accomplishment. Duct leaks up stream of the VAV’s, dirty filters and coils, plus adding more VAV’s to the system, without consideration, will also impact the system’s ability to maintain proper duct static pressure. 8.147A
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