How to Determine Makeup Air for Exhaust Fans

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It is often impractical, and sometimes contrary to local building codes and health regulations, to condition and recirculate air in certain locations within an interior space that is subject to containing fumes, smells or excess steam. In such cases, adding an exhaust fan is necessary in order to eliminate the air; likewise, make-up air must be supplied as well to prevent indoor air quality problems. How much make-up air you need to supply is dependent on the model exhaust fan being used and how the room is used.

  • Determine how much air, measured in CFMs or cubic feet per minute, is being exhausted by the exhaust fan. You can find this data within the product literature provided by the manufacturer.

  • Check with your local building official to determine whether the interior space from which you are exhausting air requires a negative or neutral pressure. This will depend on the classification of the structure and on the purpose of the interior space in question.

  • Decide how much make-up air is needed. If you need to maintain a neutral pressure, then supply an equal amount of air. If you need to maintain a negative pressure in relation to the space outside the room or structure itself, then supply 5 to 10 percent less air than is being exhausted.

Tips & Warnings

  • If there are no health or safety codes requiring otherwise, it is best to have a neutral or even slightly positive pressure within the interior space to prevent untreated, outside air from infiltrating the room.
  • The exhaust fan will not exhaust the amount of air specified by the manufacturer if it is incorrectly installed.

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References

  • "Commissioning Buildings in Hot Humid Climates: Design and Construction Guidelines"; Eds. J. David Odom and George DuBose; 2000
  • Christian Smith; HVAC Contractor; Greenwood, South Carolina
  • "HVAC Systems: Testing, Adjusting and Balancing"; SMACNA; 1993
  • Photo Credit fan blade image by robert mobley from Fotolia.com
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