Fans are used to introduce, distribute, recirculate and exhaust air in a building. Checking air velocity at various points assures that air is being distributed as expected through the ventilation system. Measurements should be made at both the supply- and return- air sides of the system.
Regular “spot” checks should be performed in different locations throughout the building to be sure that the system is performing as expected. Pay special attention to any changes in the building that may impact the Hvac system's performance. Examples include switching from cooling to heating or vice versa, remodeling, rearranging space, enlarging or reducing the area served by the system, and a change in occupancy.
Make air-velocity measurements at diffusers and registers, and at the supply and return sides of the system. Measure the ductwork and pay particular attention to sections close to dampers, transitions, elbows, branches and take-offs to make sure air is moving as expected throughout the system and that there are no impediments.
For highest accuracy, make measurements in a straight sections of the duct; roughly the equivalent of 7.5 duct diameters downstream and 3 duct diameters upstream from anything that may cause a disturbance in airflow.
Source: TSI's “Hvac Assessment Handbook”
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