Vav System - Johnson Controls VMA 1410 Overview And Engineering Manuallines

6 VMA1400 Series Overview and Engineering Guidelines Technical Bulletin

VAV System

Theory of Operation
A VAV air handling system typically consists of a single air handling
unit and multiple terminal units. Terminal units typically consist of a
damper and flow sensing probe installed in an enclosure. VAV
terminal units are also called VAV boxes. VAV systems are
predominantly single duct, but about 15% are dual duct designs.
In either case, the supply air temperature and static pressure of the
air handling unit are controlled by an AHU (Air Handling Unit)
controller, while each zone has its own VMA controller.
The air handling unit typically maintains a static pressure in the range
of 125 to 375 Pa (0.5 to 1.5 inches water column (w.c.) inside the
longest run of duct away from the supply fan. This ensures that each
VAV terminal unit has enough pressure at its inlet to deliver the
maximum required flow of air into the space. The supply temperature
is typically in the range of 7 to 16°C (45 to 60°F) for a single duct
VAV system or the cold deck of a dual duct VAV system. The hot
deck temperature of a dual duct VAV system is typically in the range
of 29 to 49°C (85 to 120°F).
VAV systems are most easily understood by first considering a
cooling-only application. As the zone temperature increases, the VAV
controller opens the VAV box damper to allow more cool air to reach
the space. The volume of air required to maintain a particular zone
temperature setpoint is dictated by the size of the space and the
internal and external heat loads. In addition, since the size of the VAV
box dictates its maximum cooling capacity, a VAV box's performance
is dependent upon the mechanical engineer's correct box sizing for
each zone.
Sometimes the size, and thus the capacity, of the VAV box may not
match the zone loads. If the installed unit is too small, insufficient
cooling results and noise may be emitted at high flow. If the installed
unit is too large, proper control may be difficult to attain, since a small
change in damper position causes a large change in airflow. Boxes can
be oversized to allow for quieter operation or reserve cooling capacity
at the expense of controllability.