In buildings that contain laboratories, fume hoods are normally used to control contaminant concentrations. Exhaust stacks with a constant exit velocity are required to make sure that dangerous concentrations do not occur in occupied areas near the building or on the roof top. To achieve constant velocity when exhaust flow rates are less than design, makeup air is introduced to the system at the inlet of the exhaust fan. Since laboratory exhaust airflow is often significantly less than the design airflow, exhaust fans consume significantly more energy than is necessary. To reduce exhaust fan energy, techniques involving multiple exhaust stacks and a variable speed drive (VSD) can be applied to laboratory exhaust systems. The potential fan energy savings depend on optimal selection of the number of stacks, the sizes of the stacks, and the exhaust system ductwork design. This paper introduces application principles, describes the optimal methods of stack sizing, and presents an example to demonstrate these methods. Published in 2005 by John Wiley Sons, Ltd.