To identify why fans do not run as expected, there are many reasons to check. But first and foremost, it is vital to know what is the accepted performance standard. As specified in the AMCA Publication 200, a range of +/- 7.5% for flow (cfm) shall be allowed for a fan that is mounted in a ventilation system. That is to say if a fan has a running speed of 1,000 cfm in a lab environment, an airflow rate of 925 to 1,075 cfm shall be provided for the fan to be installed in a ventilation system that is properly designed. If the airflow rate falls below this tolerance range, it is usually the results of differences in static pressure of the system or some mechanical problems that the fan encounters. The following are the common symptoms:

• There are obstructionsin the duct system, such as closed dampers, closed registers, dirty filters and clogged coils.
• There are obstructionsin the inlet. For example, the elbows are too close to the inlet or the walls are too close to the inlet.
• Ductsare not well designed. For example, the turning vanes are improperly designed; there are leaks in the supply or exhaust ducts.
• Problems related to the fan occur. Such as the wrong wheel running rotation, too slow fan speed, dirty or cloggedwheel and incorrectly designed clearances between inlet cone and wheel cone are incorrect.