ENGINEERING MANULA OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
411
The dotted line of Figure 10 shows a modification of this
strategy wherein the constant volume requirement is relaxed a
little to provide some VAV cooling control prior to going into
the heating/mixing portion of control. This allows a dead band
and a little VAV cooling control. This uses the same hardware
(and cost) and allows an easily adjustable minimum airflow,
up to the maximum. In this configuration, the room will operate
at the heating setpoint most of the time and the lower the
minimum airflow is set, the more time control will be in the
deadband range or at the cooling setpoint. Rooms may have
either high minimum airflow setpoints or low minimum airflow
setpoints.
Dual Duct Pressure Independent
Constant Volume ATU
Figure 11 shows a constant volume pressure independent dual
duct ATU. Modulating the hot duct damper, up to a total flow
not to exceed the maximum airflow setpoint maintains the room
temperature. A constant total airflow is normally maintained
by modulating the cold duct damper. With this configuration,
there is no deadband and comfort is maintained by mixing hot
and cold air at a constant total airflow. Hot and cold duct
temperature load reset is very important in all dual duct systems
to minimize energy waste.
Fig. 11. Dual-Duct Constant Volume Air Terminal Unit.
MAX
HEATING
LOAD
M10546
MAX
COOLING
LOAD
ZERO
AIRFLOW (CFM)
CONSTANT
AIRFLOW
SETPOINT
HO
T DUCT
AIRFLO
W
COLD DUCT
AIRFLOW
75 75
55
78
950 95000 77
AIRFLOW
SETPOINT
(CFM)
CURRENT
AIRFLOW
(CFM)
DAMPER
PERCENT
OPEN
COOLING
SETPOINT
TEMPERATURE
WALL MODULE
HOT
DUCT
COLD
DUCT
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