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繁體中文AHU Fan Maintenance: Common Problems and Troubleshooting Guide
08/19/2026Safety Before Diagnosis
Fan troubleshooting begins with safe isolation. An AHU can contain rotating parts, stored electrical energy, automatic start commands, sharp edges, hot surfaces, and pressure differences that move access doors unexpectedly. Qualified personnel should follow the site’s lockout and electrical procedures, confirm that rotation has stopped, and respect manufacturer instructions. A stopped fan may restart from a building-automation command unless all relevant energy and control sources are secured. Before opening the unit, review alarms and operating trends because they can preserve evidence that disappears after shutdown. Maintenance should never defeat guards, door interlocks, fire controls, or protective devices merely to keep the unit running.
Low Airflow
Low airflow can result from a dirty filter, fouled coil, closed damper, blocked intake, slipping belt, incorrect rotation, low speed command, damaged impeller, duct obstruction, or an inaccurate sensor. Start with the simplest observations: verify the commanded and actual fan speed, damper positions, filter pressure, and whether the fan rotates in the correct direction. Compare static pressure at logical points through the AHU and distribution system. A high fan-section pressure with low downstream airflow suggests a restriction, while low pressure and low airflow may indicate inadequate fan output or bypass leakage. Avoid increasing speed until the cause is known, because extra speed can overload equipment or hide a serious blockage.
Excessive Airflow or Pressure
Too much airflow may follow a failed pressure sensor, incorrect control scaling, a manually overridden drive, open bypass, poorly balanced branches, or a setpoint that was never reduced after commissioning. Symptoms include noisy diffusers, doors that are hard to operate, high fan energy, unstable terminal control, and coils operating outside their intended face velocity. Check the feedback value against an independent measurement and inspect the sensor tubing for leaks, water, kinks, or incorrect port connections. Review the control sequence and trend history for overrides. Reducing speed without confirming ventilation and critical-zone needs can create another problem, so changes should be measured and documented.
Vibration and Imbalance
Vibration may be caused by dirt on the wheel, missing balance weights, damaged blades, loose fasteners, worn bearings, shaft misalignment, belt problems, a soft support, or structural resonance. Inspect for visible buildup and damage only after safe isolation. Check mounting hardware, isolation devices, bearing condition, and whether flexible connectors are pulling the assembly out of position. The vibration pattern and frequency can help trained technicians separate rotational imbalance from misalignment, bearing defects, looseness, or aerodynamic instability. Repeatedly balancing a dirty or deteriorating wheel is not a lasting repair. The underlying contamination source, drainage issue, filter bypass, or operating instability must also be corrected.
Bearing and Mechanical Noise
Rumbling, grinding, squealing, or knocking should be investigated promptly. Bearing distress can follow lubricant problems, contamination, electrical damage, excessive belt tension, misalignment, or operation beyond design conditions. Lubrication practices must match the bearing manufacturer; both under-lubrication and over-lubrication can be harmful. Temperature readings are most useful when trended under comparable loads rather than judged from one isolated value. For direct-drive fans, motor bearings may be part of an integrated assembly. For belt-driven equipment, inspect sheaves, keys, shafts, guards, and alignment. Do not use a louder sound as the only threshold for action, because deterioration may progress before it becomes obvious.
Belt and Pulley Problems
Belts that are loose can slip, reduce airflow, generate heat, and leave dust. Belts that are too tight can overload bearings and shafts. Misaligned sheaves cause edge wear, vibration, and shortened life. Inspect belt condition, matched sets, tension, pulley grooves, fasteners, and alignment using appropriate tools and procedures. Replacing one belt in a worn multi-belt set can produce uneven load sharing. Oil or chemical contamination should be traced to its source rather than wiped away repeatedly. After service, reinstall guards and verify rotation and airflow. A recurring need for retensioning may indicate worn grooves, structural movement, incorrect belt selection, or an unresolved alignment issue.
Motor, VFD, and EC Fan Faults
Electrical faults may appear as failure to start, unexpected trips, unstable speed, overheating, communication loss, or reduced output. Review the exact alarm code and event history before cycling power. Check supply conditions, fuses, isolators, terminals, control signals, grounding, motor data, and cooling paths within the limits of technician qualification. A VFD trip can be a symptom of mechanical overload or a restrictive system, not merely a drive problem. EC fans add integrated electronics, so replacement and diagnostic procedures may be product specific. Condensation, excessive ambient temperature, conductive dust, or damaged communication wiring can affect either technology. Preserve parameter backups before changing settings.
Unstable Operation and Surging
Pulsating airflow, pressure oscillation, or a repeated rise and fall in speed can come from control-loop tuning, sensor location, rapidly moving dampers, or fan operation near an unstable region. Examine trends of command, speed, pressure, and terminal positions on the same timeline. If the command oscillates first, the control loop or feedback is likely involved. If pressure fluctuates despite a steady command, the fan or system may be aerodynamically unstable. Do not simply slow the control loop until all oscillation disappears; excessive delay can impair pressure control. The durable solution may require a different operating range, setpoint, sensor location, minimum speed, or duct modification.
Dirt, Corrosion, and Moisture
Deposits change blade shape, reduce efficiency, and create imbalance. Corrosion weakens components and can make future cleaning more difficult. Moisture may come from coil carryover, drain problems, casing leakage, humidification, outdoor-air ingestion, or condensation on cold surfaces. Identify the source before cleaning and repainting. Use cleaning products and methods approved for the wheel, coating, motor, and electronics; high-pressure spray can drive water into bearings or connectors. Inspect drains, traps, eliminators, seals, and access-door gaskets. In contaminated industrial or kitchen exhaust service, maintenance frequency should reflect the actual buildup rate and fire, hygiene, or process risks.
Building a Preventive Maintenance Program
A useful program combines scheduled inspection with condition data. Record fan speed, airflow, pressure, power or current, vibration, bearing condition, filter differential pressure, belt state, alarms, and cleaning observations. Baselines should be taken after commissioning or a verified repair. Trend changes under comparable operating conditions and define who reviews them. Keep replacement parts, configuration files, wiring diagrams, and fan identifiers accessible. After any repair, confirm guards, access doors, rotation, speed limits, control response, and measured airflow. Good maintenance is not a sequence of emergency fixes; it is a disciplined process that catches loss of performance before it affects comfort, energy use, indoor air quality, or production.
