Constant speed fan exhaust systems are the most popular exhaust system in laboratory buildings since they satisfy building architectural requirement, and have low initial costs with reliable performance. The constant speed fan exhaust system maintains constant stack exit velocity to prevent toxic exhaust air downwash back to occupied areas near the building or on the roof top. To achieve constant velocity when the exhaust airflow is less than the design value, makeup air is introduced to the system at the inlet of the exhaust fan. Since the laboratory exhaust airflow is often significantly less than the design airflow, exhaust fans consume significantly more energy than is necessary. Multi-stack and Variable frequency drive techniques can reduce both the make-up airflow rate and fan energy in the constant speed fan exhaust system retrofit. This paper introduces the application principles, describes. the optimal methods of stack sizing, and presents an example. The application example showed that the average annual fan power is 50% of the design fan power for a two-stack system, and 47% for a three-stack system.