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26 TRG-TRC014-EN
notes
period two
Components of a VAV System
VAV Terminal Unit Controls
Next we will explore the control of VAV terminal units. Before discussing the
commonly-available control technologies, it is important to first understand a
fundamental control concept: pressure-dependent versus pressure-
independent control.
The modulating device in a VAV terminal unit changes the air resistance of the
supply duct path to the space. As terminal units modulate, the static pressure
within the duct system changes. The airflow that passes through the terminal
unit depends not only on the position of the modulation device but also on the
static pressure at the inlet. With the modulation device in a fixed position, a
higher static pressure in the upstream duct will “push” more air through the
terminal unit. Because the inlet static pressure constantly changes due to the
modulation of the terminal units in the system and the resulting variation of
VAV Terminal Unit Controls
Figure 39
Upstream Pressure Affects Airflow
VAV terminal unit
VAV terminal unit
(identical damper positions)
(identical damper positions)
static pressure
static pressure
in supply duct
in supply duct
0.70 in. H
0.70 in. H
2
2
O
O
[175 Pa]
[175 Pa]
0.80 in. H
0.80 in. H
2
2
O
O
[200 Pa]
[200 Pa]
static pressure
static pressure
in supply duct
in supply duct
1000
1000
cfm
cfm
[0.47 m
[0.47 m
3
3
/s]
/s]
1300
1300
cfm
cfm
[0.61 m
[0.61 m
3
3
/s]
/s]
Figure 40