There are numerous reasons why fans fail to perform as specified. But first it is important to understand what defines acceptable performance. According to AMCA Publication 200, a fan installed in a ventilation system should expect a tolerance of +/- 7.5% for flow (cfm). In other words, a fan that produces 1,000 cfm in a lab environment should provide a volume flow rate of 925 to 1,075 cfm when installed in a properly designed ventilation system. Volume flow rates falling below this range are typically the result of variances in system static pressure or mechanical problems with the fan.

Common symptoms include:

• Obstructions in the duct system - closed dampers, closed registers, dirty filters, clogged coils
• Obstructions in the fan inlet - elbows too close to the inlet, walls too close to the inlet
• Duct design - improperly designed turning vanes, leaks in supply or exhaust ducts
• Fan related - impeller running backwards, fan speed too low, impeller dirty or clogged, clearances between inlet cone and wheel cone are incorrect

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