What is the difference between constant air volume vs. variable air volume hoods?
Generally, fume hoods fall into one of two categories: constant air volume (CAV) and variable air volume (VAV). The construction of CAVs is relatively simple; they are simply boxes that are ducted to fans that exhaust air to the atmosphere, and typically do not have any mechanism to adjust the volume of air exhausted. Capital costs for CAVs tend to be less than VAVs. VAVs, on the other hand, have additional devices that allow the volume of exhausted air to vary in response to sash height (Box 1), occupancy, or another factor. VAVs can modulate air flow by adjusting fan speed (variable speed drives) or by varying the position of dampers in the duct or the VAV “box”. VAVs may also improve safety by narrowing the range of face velocities experienced at different sash heights. VAVs reduce operational costs by permitting reduced airflow, and thereby conserving energy. At the University of Toronto (UofT) a large number of VAVs are installed in the John and Edna Davenport Chemical Research Bu
