Fan Malfunctions: Five Common Issues and Their Causes

Fan Malfunctions: Five Common Issues and Their Causes

3. Improper fan performance. Causes of insufficient airflow include the impeller being installed or running backward, incorrect blade angle settings, failure to cut off, the impeller not being centered with the inlet shaft collar, slow fan speed, a dirty or clogged inlet, improper operating clearance, or improper fit between the inlet cone and the impeller. Insufficient airflow entering the duct system indicates that the actual system offers greater resistance to airflow than expected. Other possible causes include closed dampers or regulators, loose insulation lining in ducts, or leaks in supply ducts. If airflow is insufficient at a filter or coil, check for dirty or clogged filters, replace filters with pressure drops exceeding specifications, or install replacement coils with smaller fin spacing. Airflow issues caused by poor inlet conditions may be due to elbows, cabinet walls, or other obstructions. Inlet obstructions will cause the system to become more restricted but will not result in increased negative pressure readings near the fan inlet. Increase the fan speed to offset the effects of the restricted inlet, but do not exceed the manufacturer’s recommendations. Sudden expansion or contraction at the fan outlet, or bends in the ductwork, can result in poor outlet conditions and restricted airflow. If installing a straight duct section at the fan outlet is impractical, increase the fan speed to overcome the pressure drop, but do not exceed the manufacturer’s recommended speed range. Excessive airflow is caused by an oversized duct system, dampers set to bypass, missing filters, excessive fan speed, low system resistance, or open passages. Keep in mind that air density decreases at high temperatures or at high altitudes. High airflow indicates that the system’s flow resistance is lower than expected. When airflow is low, the impeller may be installed backward or running backward. Other possible causes include improper blade angle settings, missing cut-offs, fan speed that is too slow, a dirty or clogged impeller or inlet, improper operating clearances, or improper inlet blade or damper settings. It is also possible that the system’s flow resistance is greater than designed, dampers are closed, coil fin spacing is too tight, or filter replacement is too restrictive.