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What is covered in this document?

This document is part of a series of documents on industrial ventilation:

  1. Introduction
  2. Units and Measures
  3. Ducts
  4. Fans
  5. Hoods
  6. Air Cleaning Devices
  7. Installation and Maintenance (general)
  8. Troubleshooting
  9. Glossary of Common Terms

What are some general troubleshooting tips?

Most of ventilation system problems can be avoided by periodic maintenance and checked by taking measurements of air speed or pressure in the system. Airflow at the hood can be visually checked with inexpensive smoke generators (smoke tubes) or measured with air velometers. Ventilation specialists may be needed to fix or redesign more complicated ventilation problems.

The following simple checklist can be used to evaluate the functioning of ventilation system without extensive measurements or expert help.

Observation Yes No

Is the fan belt broken or slipping?


Is the fan wired backwards (reversed polarity)?


Is duct clogged with dust?


Is there holes, cracks or openings in the ducts?


Is the air cleaner clogged?


Are any dampers in the duct closed?


Is there insufficient makeup air?


Have ducts been changed to include more length, more or sharper bends, or abrupt diameter changes?


Have additional hoods and ducts been added? (Without proper airflow balancing, some hoods in a multiple system may have inadequate flow or the fan may be too small to handle the additional resistance.)


Has the contaminant source been moved further away from the hood opening?


Is the canopy hood located as close to the source as possible without letting the employees work over the source?


Is an access to enclosing hoods provided?


Is more contaminant being generated at the source?


Are cooling fans causing cross drafts?


Have employees modified the hood because it interferes with their job tasks?


What are some troubleshooting tips for hoods, ducts, air-cleaning devices & fans?

Exhaust Hood
Problems Possible Cause(s)
Low Capture Velocity
  • Hood located too far away from process or operation generating point.
  • Cross drafts or turbulence close to hood.
  • Blocked duct work.
Reduced Face Velocity
  • Unauthorized enlargement of the opening of an existing hood. The face velocity at the hood will decrease resulting in the escape of contaminants held back by the original face velocity.


Problems Possible Cause(s)
Constant Plugging
  • Ducts handling particulates with inadequate transport velocity (speed).
  • Flexible ducts used in place of rigid ducts (results in extra friction loss).
  • Condensation of salts.
  • Dented ducts.
  • Elbows with sharp turns.
  • Holes in ducts.
  • Disconnected or broken branch connections.
  • Closed or partially closed dampers (blast gates).
  • Addition of hoods and branches to system without making adjustments to rebalance the ventilation system.


Air Cleaning devices
Problems Possible Cause(s)
Frequent clogging
  • Improper filter type and/or installation.
  • Improper bag cleaning cycles.
  • Water intrusion into the filter.
  • Filter hopper not continuously emptied and cleaned.
  • Improper "clean start-up procedure" for new bags.
Visible dust in baghouse
on the clean air side or
in exhaust stack
  • Bags installed improperly.
  • Torn or damaged bags.
  • Leakage between bags and housing.
  • General filter fabric failure.
Sudden increase in
pressure drop
  • Excessive dust loading due to lack of maintenance and scheduled cleaning.
Dirty re-circulated air
  • Dirty filters.
  • Air bypassing filter section (rip or tears in filter).
  • Dirty air handling cabinet housing.
  • Exhaust stacks placed close to supply air intakes.
Visible mould or slime
  • Drain pans not operating properly.
  • Drain pans overflowing.


Problems Possible Cause(s)
  • Out of balance fan impeller.
  • Material on fan blades.
  • Loose fan housing or foundation bolts.
  • Fan running backwards.
  • Vibrating ducts.
  • Foreign material in fan housing.
Insufficient air flow
  • Fan running backwards.
  • Fan speed too slow.
  • Dirty fan blades.
  • Actual system has more resistance than designed for.
  • Dampers closed.
  • Leaks in duct work.
  • Dirty or clogged filters in air cleaning device.
  • Obstructed fan inlets causing system effects (No straight duct runs at fan inlet or outlet).
  • Fan not getting adequate make-up air.
Excessive air flow
  • Access door open.
  • Filters not in place or tears in filters.
  • System resistance low.
  • Fan speed too fast.
Fan does not operate
  • Blown fuses.
  • Broken belts.
  • Loose pulleys.
  • Electricity turned off .
  • Fan impeller touching housing.
  • Wrong voltage.
  • Motor too small (overload protector had broken circuit).
  • Low voltage.
  • Fan load too large for motor.
  • Seized bearing.

Document last updated on January 10, 2008


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