The desired result of all space pressurization control systems is to control the infiltration into or the ex filtration out of a space. Space pressurization control strategies can be divided into two major categories: passive and active.

For constant-volume laboratories a passive method involves simply balancing the system so that the desired space pressurization is achieved. This method has serious limitations which should be considered carefully before choosing to design a constant-
volume system with passive space pressurization “control.” This type of system will work only if: 1) all fume hoods remain on and at constant speed or volume at all times, 2) no exhaust sources (i.e. hoods) are added or removed, 3) the offsets are large
enough to mask changes in exhaust and supply system performance caused by filter loading, etc., 4) the system is tested and balanced frequently to design conditions, and 5) the system is adequately maintained. If you cannot guarantee (or even desire) all
these restrictions then this design approach is inappropriate for your application. An active method for use in a constant-volume laboratory involves the utilization of pressure-independent, constant-volume control devices in the exhaust and supply ducts

VAV labs require active methods to control space pressure due to the continuously changing exhaust volume from the fume hoods and other exhaust sources. Active VAV space differential pressure control methods may be subdivided into two types: pure
differential pressure measurement/control (ÆP) and differential volume or flow-tracking (ÆV).