Dual-Duct Atu; Dual Duct Pressure Independent Vav Atu - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS

DUAL-DUCT ATU

In a dual-duct air handling system, supply air is divided at
the central fan and hot air and cold air flow through separate
ducts throughout the building. Two of the many DDC dual-
duct control configurations are included here.
A typical original dual-duct ATU draws hot and cold air from
respective ducts, mixes the air as directed by a sensor in the
controlled space, and discharges the mixed air into the space.
This basic dual-duct mixing system is not economical because
air is cooled and heated much of the year. DDC systems are
still sometimes used with digital hot and cold duct load reset to
minimize the energy waste during mixing periods. Adapting
dual-duct systems to VAV control has resulted in significant
energy savings without discomfort.
INCHES OF 78
WATER
1.4
55
350
MINIMUM
AIRFLOW
SETPOINT (CFM)
SETPOINT (CFM)
COOLING
MAX
HEATING
MAX
MIN
ZERO
ENGINEERING MANULA OF AUTOMATIC CONTROL
HOT
DUCT
COLD
DUCT
1150 925
MAXIMUM
MAXIMUM
COOLING HEATING
AIRFLOW
AIRFLOW
SETPOINT (CFM)
FULL 0
HEATING
LOAD
HEATING SETPOINT
Fig. 10. Dual-Duct Pressure Independent VAV Air Terminal Unit.

Dual Duct Pressure Independent VAV ATU

Figure 10 shows a dual-duct pressure independent ATU. The
airflow sensor in the outlet of the ATU controls the cold duct
inlet damper in the cooling mode from the cooling maximum
airflow to the minimum ventilation airflow, through the dead
band, and to the heating setpoint. As the heating load increases,
the hot duct damper modulates open to control the supply
airflow from zero to the maximum heating value. This same
sequence may be achieved with two airflow elements, one in
each primary duct
75
00 77 943
DAMPER CURRENT
PERCENT AIRFLOW
OPEN (CFM)
0
COOLING
DEDBAND
LOAD
(ADJUSTABLE)
COOLING SETPOINT
410
WALL MODULE
TEMPERATURE
COOLING
SETPOINT
75
-3.0
HEATING
DEADBAND
FULL
M10545