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TRG-TRC014-EN 17
period two
Components of a VAV System
notes
During the heating mode, the primary airflow must often be greater than the
minimum setting of the unit for the cooling mode. When warm, buoyant air is
supplied from the ceiling, a certain velocity is required to effectively deliver
it down to the occupied portion of the space. Increased airflow may also be
needed to meet the minimum requirement for proper operation of the
heating coil.
VAV reheat is more efficient than the constant-volume reheat system discussed
earlier because heat is provided at reduced airflow and only when required.
Another method that a single-duct VAV terminal unit can employ to heat a
space is to mix warm plenum air with cool primary air. Because this method
uses a small fan to draw warm air from the plenum and mix it with the primary
air, they are called fan-powered terminal units.
They come in two configurations, parallel and series.
cool
cool
primary air
primary air
warm
warm
plenum air
plenum air
Fan-Powered Terminal Units
series
parallel
Figure 27