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18 TRG-TRC014-EN
notes
period two
Components of a VAV System
A parallel, fan-powered terminal unit consists of a primary airflow
modulation device and a small, integral, constant-volume fan packaged to
provide parallel airflow paths.
In the cooling mode, the unit is controlled in the same manner as the cooling-
only unit. Primary airflow is reduced as the cooling load in the space decreases.
When the space cooling load drops below the minimum primary airflow setting
for the unit, overcooling the space, the small fan activates to mix warm plenum
air with the cool primary air. This increases the total airflow to the space and
creates a warmer supply air condition. If additional heating is required, with the
terminal fan on, a heating coil can be used to further warm the supply air.
Operating the fan is a form of energy recovery. It tempers the supply air with
heat from the building and lights carried by the return air, rather than with
“new” energy, thus delaying the start of the heating coil.
Parallel, Fan-Powered
primary
primary
air
air
supply
supply
air
air
terminal
terminal
mixing fan
mixing fan
plenum
plenum
air
air
Figure 28
100%
0%
primary air
primary air
plenum air
plenum air
space load
design
cooling load
design
heating load
% airflow to space
minimum
primary
airflow
maximum
primary
airflow
heating
heating
tempering
tempering
Parallel, Fan-Powered
Figure 29