The variable air volume (VAV) system provides a variable quantity of air to a zone, in response to a signal from the thermostat. At any given moment, the ft3 per minute of air delivered by the main air handler varies according to the settings of all the VAV boxes downstream. These metering devices operate best when they see a constant static pressure, so the AHU fan must be made to maintain a steady static pressure at the fan outlet.
The most basic control means is to just operate the fan at a constant speed, using inlet (or outlet) vanes to restrict the return airflow so that a constant static pressure happens downstream of the air handler. This is an effective method that uses the pneumatic signal from a static pressure gauge to operate adjustable dampers.
A variation of this system can reduce the fan energy usage. The static pressure gauge controls the fan speed via a variable-speed drive, rather than with mechanical vanes. Since the energy used by the fan varies as the cube of the fan speed, even a slight reduction in fan speed can reduce energy use by a great deal.
Either system is based on the signal from a static pressure sensor. Most smaller VAV systems use inlet vanes for airflow control, as the much more expensive VFDs are economical only on larger systems.
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Excerpt from Energy Conservation Projects.