Modern-day technology provides many tools for accurate and reliable fan and system design. Gone are the days when cheap energy allowed rules of thumb and large safety factors to be part of the design process. However, even with modern technology, instances occur when the actual application does not perform as expected.

Troubleshooting is the effort to identify and resolve differences between what was expected and what actually happened.

This article is a guide for identifying the most common problems encountered in fans and air-handling systems. This is often an "art" and not a "science" because many problems are subtle and hard to diagnose. It often takes a lot of knowledge, inquisitiveness, common sense, mechanical aptitude, and years of experience to diagnose the cause of some very subtle problems. A professional consultant or troubleshooter may ultimately be required to identify and solve the worst problems.

The intent of this article is to point people in the right direction.

Safety Considerations

Fans and air-handling systems come in many sizes, shapes, and complexities. It is critical to realize the limitations of any investigation to ensure the safety of all personnel and the safe operation of installed equipment. Operating a piece of equipment or system when there are obvious mechanical, electrical, or aerodynamic instabilities requires extremely good judgment, and investigations should be conducted only by qualified personnel. Catastrophic failure resulting in death or serious physical damage can occur when rotating equipment is involved. Physical inspections should be made only when the fan and system are shut down and locked out, both electrically and mechanically, so that windmilling cannot occur.

It is strongly recommended that AMCA Publication 202, "Troubleshooting" and AMCA Publication 410, "Recommended Safety Practices for Users and Installers of Industrial and Commercial Fans" be thoroughly read prior to any investigation. Reference should also be made to the manufacturer's installation and maintenance literature. OSHA requirements for guarding should also be reviewed. Proper attire such as safety shoes, hard hat, safety glasses, no ties or loose fitting clothing, safety harnesses, etc., must be worn. In no case should the troubleshooter become part of the problem due to a lapse in safety.

Required Information

Equipment identification- It is vitally important that the equipment in question be properly identified along with other related information. This allows the equipment manufacturer's records to be accessed, allowing a starting point for an initial evaluation of the equipment at the time it left the factory. The following information is necessary:

  • Customer name;
  • User;
  • Job site location;
  • Fan serial number or order number;
  • Fan model; and
  • Specific fan designation, if several fans are involved.
Detailed description of the problem- It is essential for those involved to properly describe the problem in as detailed and clear a manner as possible. Information is often transferred through several different people and "word of mouth" is normally not sufficiently accurate. Complete descriptions including noises are encouraged as they can be very helpful in identifying a problem. As many details as possible should be included. As an example, if a system is low in airflow, related rating parameters such as pressure, power, speed, elevation, and temperature should also be provided.

A statement of the actual airflow compared to the desired airflow should be included. Test measurements and their locations are very useful since 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 of the simpler problems can be solved up front by conducting a thorough inspection. This is normally the user's responsibility prior to getting others involved. The following checklist from AMCA 202 contains 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 does not "suck in," leak, or become short-circuited by having the fan support ductwork or other parts of the system.
  • Check that all driveline components such as bearings, couplings, V-belts, motors etc., 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.
  • Check that the fan wheel is properly aligned with the inlet bell and housing, is free to turn, and that when momentarily energized, it will turn in the right direction.
  • Check the fan and system for any obstructions, build-up, leaks, missing parts, etc.
  • Run the fan at full speed. Verify that the fan is running close to the design speed. Determine whether the fan is running smooth and that the bearings are not running hot. Obtain a power measurement to make sure the fan is not 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, it will serve as a beginning point of any evaluation.

Contact person - If there is a problem, it is necessary to designate an individual as a contact person who will have continuing intimate knowledge of the fan and system status, knows the problem, and has already been tried to solve it. This person is to serve as the contact person and liaison for others who may be required to visit the job site. This person's name, title, address, and phone number should be readily available, and he or she should be kept abreast of all actions that may be contemplated. Problem priority - There must be some determination as to the seriousness and timeliness of the problem resolution. Many misunderstandings occur when a priority has not been established. It is necessary that all of those involved in the resolution recognize that a problem exists and that an amiable plan of action and solution is usually in the best interest of everyone.

Problem Categories

As previously stated, this article is intended to point the troubleshooting process in the right direction. Required information has been outlined. It is now necessary to identify the nature or category of the specific problem. The following four categories narrow the focus of attention and speed up the evaluation process.

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

Noise - This applies to any problem in which the ears are the main sensor. Noise and vibration are similar in that they both have amplitude and frequency, but noise is a much lower amplitude and energy content, and is measured in dB referenced to Watts. Generally speaking, noise has a much wider frequency range and a higher upper limit than vibration (63Hz to 10 KHz).

Vibration - This applies to any problem in which the hands or touching are the main sensor. Amplitude is large when there is a problem. It has a much greater energy content with a smaller frequency range (3Hz to perhaps 500 Hz). Premature failure - Premature failure applies to anything whose life does not meet that which was expected. The term "failure" does not necessarily mean a catastrophic failure such as when something "blows up," but a length of time considered as being the useful life of the component.


Satisfactory applications occur when all aspects of the installation are in harmony with each other. Proper operation, constant monitoring, and maintenance are also part of the equation. Troubleshooting must be employed when one of these areas becomes a problem.

Noise is generally considered low-quality, unwanted sound. The ears sense noise, whereas vibration is sensed by feel or touch. Sources of noise can usually be identified by some form of characteristic sound to which we can relate. Words such as tone, rattle, pitch, steady or unsteady, and intermittent are examples. These characteristic words help to define whether the source of the noise is aerodynamic, mechanical, or electrical.

Aerodynamic generated noise is characterized by a continuous broadband frequency spectrum with a superimposed tone. The tone is typically objectionable when it becomes 4-6dB 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 very objectionable when system effects and various controls cause it to rise higher than normal. Additional causes include turbulence, high velocities, and instabilities due to pulsation and surge.

Mechanically generated noise has a different sound quality and characteristic. It 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 VFD and motors can cause a substantial increase in the motor noise due to imperfect sine wave simulation.

There are many different sources of vibration. One of the most difficult tasks in troubleshooting fans and systems is the systematic identification of vibration characteristics (amplitude, frequency, location, direction, units of measurement) as a function of operating point location on the fan curve and control settings. Identification of the source can be extremely difficult, in some cases requiring the services of a professional troubleshooter.

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.

It is obvious premature failure general considerations: It is obvious that a component that physically fails and flies apart upon start-up is a premature failure. However, premature failures also occur when fans and components do not satisfy their expected life. This is hard to quantify because very few records are kept, and once a fan is installed, it is easily forgotten. The best prevention against premature failure is a good conscientious preventative 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.

In general, the equipment life should be consistent with the application. As an example, HVAC equipment may be expected to last 10 years, industrial equipment about 15 years, and power plant equipment about 30 years. This means that the equipment itself, with proper maintenance, should still be around after these time frames. AMCA