Fan and System Troubleshooting: A Guide to Characteristic Field Problems

Identifying the most common problems encountered in fans and air handling systems can be an art rather than a science. Fans and air handling systems come in many sizes, shapes, and complexities, and their maintenance issues are often subtle and hard to diagnose.

Before beginning any service, it’s critical to ensure the safety of all personnel and the safe operation of installed equipment.

First and foremost, when doing physical inspections, make sure that the fan and system are shut down and locked out both electrically and mechanically so that windmilling can’t

We also strongly recommend reading AMCA (The Air Movement and Control Association
International, Inc.) Publication 202, "Troubleshooting" and AMCA Publication 410,
"Recommended Safety Practices for Users and Installers of Industrial and Commercial Fans" prior to any investigation.

Finally, read the manufacturer’s installation and maintenance literature, and review OSHA requirements for guarding. Wear proper attire such as safety shoes, hard hat, safety glasses, no ties or loose fitting clothing, and a safety
harness. In no case should the troubleshooter become part of the problem as a result of a lapse in safety.

Required Information

Equipment Identification — Ensure that the equipment being checked is properly identified, and that you have access to the manufacturer’s records. This gives you a starting point for an initial evaluation of the equipment at the time it left the factory. Make sure that you also have the following information:

  • Customer Name
  • User
  • Jobsite location
  • Fan serial number or order number
  • Fan model
  • Specific fan designation if several fans are involved.

Detailed description of the problem — Describe the mechanical problem in as
detailed, clear manner as possible. Include information such as:

  • if the system is low in airflow
  • parameters such as pressure, power, speed, elevation, and temperature
  • actual airflow compared to the desired airflow
  • test measurements and their locations.

These parameters are quite useful because the measurements themselves are often a clue to the problem.

Initial fan and system inspection — Most problems occur during the start-up phase of any installation. Many simple ones can be solved up front by conducting a thorough inspection. The following checklist from AMCA 202 lists the items to be inspected:

  • All fan parts and accessories should be installed, aligned, and operational.
  • Check all tie down bolts so that the fan is firmly held in place on its foundation.
  • Check all ductwork connections so that flexible material doesn’t "suck in," leak, or become short-circuited because the fan is supporting ductwork or other parts of the system.
  • Check that all driveline components such as bearings, couplings, v-belts, and motors are aligned and properly tensioned. Make sure all v-belts are matched, and that bearings have been tightened to the base and to the shaft. Check that bearings are properly lubricated with the proper type and amount of grease.
  • Ensure that the fan wheel is properly aligned with the inlet bell and housing, is free to turn, and turns in the right direction.
  • Check the fan and system for any obstructions, build-up, leaks, or missing parts
  • Run the fan at full speed, verifying that it’s running close to the design speed. Also determine whether the fan is running smoothly and that the bearings aren’t running hot. Obtain a power measurement to make sure the fan isn’t overloading the motor.
  • Let the fan run for 24 hours. Recheck all of the items listed above once again, particularly the v-belt tension.

The results of this initial inspection should be kept on record for future reference. If a problem does occur later on, it will serve as a beginning point of any evaluation.

Contact person — Designate a point of contact at the facility and keep his or her name, title, address, and phone number readily available. This person should have continuing knowledge of the fan and system status. You should also keep this person abreast of all potential repairs or replacements.

Problem priority — If repairs need to be made, determine how serious the problem is, how soon it needs to be resolved, and then plot a course of action that is agreed upon by all persons involved.


Fan and air handling systems function best when all aspects of the installation are in tune with each other. Therefore, with proper operation, constant monitoring, maintenance, and sharp troubleshooting skills, your customers and their equipment will both be in perfect harmony.

Problem Categories

When troubleshooting fans and air handling systems, keep the following four categories in mind to narrow the focus of attention and speed up the evaluation process:

Aerodynamic performance -— This applies to the five rating parameters of flow, pressure, speed, power, and density and how they compare to their respective design quantities.

Noise — The source of the noise can be aerodynamic, mechanical or electrical:

  • Aerodynamically generated noise is characterized by a continuous broadband frequency spectrum with a superimposed tone. The tone is typically objectionable when it becomes 4 to 6dB louder than the rest of the spectrum. The tone can be the blade frequency, which is a function of the fan type. It can become particularly bothersome when system effects and various controls cause it to rise higher than normal. Additional causes include turbulence, high velocities, and instabilities as a result of pulsation and surge.
  • Mechanically generated noise has a metallic sound caused by metal-to-metal contact. This contact may be constant or intermittent.
  • Electrically generated noise is a function of motors, relays, controls, or unbalanced line voltages into the motor. Sometimes improperly matched variable frequency drive (VFD) and motors can increase motor noise because of imperfect sine wave simulation.

Vibration — Vibration in housings and ductwork is most often aerodynamically generated. This is a forced vibration in which the energy and characteristics of the airstream are large enough to cause sympathetic vibration in the housing and ductwork. Turbulence, pulsation, and the blade frequency tone are examples of forced vibration due to aerodynamics.

Vibration can also be the result of a resonance. This occurs when the natural frequency of a duct or housing panel coincides with a specific aerodynamic excitation such as rotating stall, vortex shedding, or the blade frequency, if it is strong enough.

Mechanically generated vibrations occur from unbalance, resonance, looseness, and rubbing. Electrically generated vibrations result from torsional fluctuations, eddy current induced fields, and improper wiring.

Premature failure — The best prevention against premature failure is a good, conscientious preventive maintenance program that includes inspections and the recording of vibration levels. Repairs should take place at the first sign of a problem, and not after damage has occurred to other parts.

This article was excerpted from the publication "Fan and System Troubleshooting — A Guide to Characteristic Field Problems," from Greenheck. Go to for the complete guide.

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