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Air Velocity Basics

Why check Air Velocity?

Fans are used to introduce, distribute, recirculate andexhaust air in a building. Checking air velocity periodicallyat various points assures that air is being distributed asexpected through the ventilation system. Measurementsshould be made on both the supply and return air sides ofthe system. Air movement or velocity has an impact onperceived comfort by occupants.

When would you check Air Velocity?

Regular "spot" checks should be performed in different locationsthroughout the building to be sure that the system isperforming as expected. Special attention should be paid whenever something in the building changesthat may impact the HVAC system's performance. Examples include switching over from heating tocooling, remodeling, rearranging a space, enlarging or reducing the area being served and adding orsubtracting people.

Where would you check Air Velocty?

Air velocity measurements should be made at diffusers or registers both on the supply and return sidesof the system. Measurements should be made in the ductwork, paying particular attention to sectionsclose to dampers, transitions, elbows, branches and take-offs to be sure air is moving as expectedthroughout the system and that nothing is impeding air movement. For highest accuracy, measurementsshould be made in a straight section of duct roughly the equivalent of 7.5 duct diameters downstreamand 3 duct diameters upstream from anything that may cause a disturbance in air flow.

Copyright © 2007 by TSI Incorporated

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