The safety ramifications of using diversity to size the laboratory HVAC system must be thoroughly considered. Diversity should not be used to size the exhaust fan capacity because of overriding safety concerns. Total installed fume hood and specified exhaust capacity must be provided. [Hitchings and Shull, 1993; Lentz and Smith, 1989]
Energy efficiency results from the exhaust system arrangement. For instance, a VFD on the exhaust fan can normally operate at the lower volume rate that is indicated by the diversity factor. The exhaust stack would be designed to maintain the required discharge velocity at this "diversified" speed. During an emergency situation, the VFD would go to full speed with an automatic bypass to provide the total exhaust system capacity, and a resulting increase in the stack discharge velocity. Instead of a VFD, a multiple-fan installation can also be arranged. The difference between the total installed exhaust capacity and the "diversity capacity" provides a safety factor.
Diversity factors should be used only for mechanical systems supporting heating, cooling, humidification, and other non-safety functions, such as air distribution duct systems, central fan systems, and primary heating and cooling systems. A diversified exhaust rate should never be used when sizing terminal air distribution units serving any individual laboratory. [Hitchings and Shull, 1993; Lentz and Smith, 1989]
Facility vs. local diversity