Daikin IM 696-4 13
Field Wiring
VAV Box Output
The VAV Box Output (MCB-B012) supplies 24 VAC to
terminal 118 on
the field terminal block (TB2) when the output
is on. To use this signal, the coil of a field supplied and
installed 24 VAC pilot relay must be wired across terminals
118 and 117 (units with inlet guide vanes) or 119 and 117
(units with VFDs) on TB2. When the output is energized, 24
VAC is supplied from the T3 control transformer through
output relay MCB-B012 to energize the field relay. Refer to
the as-built wiring diagrams or to “Auxiliary VFD control” on
page 48.
CAUTION
The VAV Box Output (MCB-BO12) is designed to coordinate
unit operation with VA
V box control. Field use of this output is
optional; however, it is highly recommended, especially for
VAV systems that have heating capability (unit or duct
mounted).
The following are application guidelines for fou
r basic heating
configurations. For all of these configurations, the VAV Box
Output (MCB-BO12) is off for an adjustable time period after
unit start-up (default is 3 minutes). During this period (the
Recirc operating state), heating and cooling is disabled, and
the outside air damper is held closed. The fans circulate
building air and equalize space, duct, and unit temperatures.
Cooling Only Units
For cooling only VAV systems, the VAV Box Output can
override zone thermostat control and drive the VAV boxes
fully open to facilitate air circulation during the Recirc
operating state. During this time, the VAV Box Output is in the
OFF (or heat) position (field-installed pilot relay
de-energized).
VAV units have a “post heat” control feature that
forces the
discharge air v
olume to a minimum before turning on the VAV
Box Output when the Recirc operating state is complete. Post
heat operation prevents excessive duct static pressure that
could otherwise occur when the zone thermostats regain VAV
box control.
1
When the unit is not in the Startup or Recirc operating state
and “post heat” is not active, the VAV Box Output is in the ON
(or cool) position (field relay energized) so that the zone
thermostats control the VAV boxes.
The field-supplied fan operation and VAV box relay contacts
can be wired in series so that the boxes open when the uni
t is
not operational.
Cooling Only Units with Field Supplied Heat
For VAV systems with cooling only rooftop units and duct
mounted reheat coils, the VAV Box Output can override zone
thermostat control and drive the VAV boxes fully open when
heating is required. If necessary, the MicroTech II controller
energizes the fans for night setback and morning warm-up
heating operation. When this occurs, the unit enters and
remains in the UnocFanO or MWU operating state until heat is
no longer required. The temperature control sequences are the
same as those for units with factory-supplied heating
equipment. While the unit is in these states, the VAV Box
Output is in the OFF (or heat) position (field-supplied pilot
relay de-energized).
VAV units have a “post heat” control feature that
forces the
discha
rge air volume to a minimum before closing the VAV
box output when the heating period is complete. “Post heat”
operation prevents excessive duct static pressure that could
otherwise occur when the zone thermostats regain VAV box
control.
1
When the unit is not in the Startup, Recirc, or any heating
operating state and “post heat” is not active, the VAV Box
Output is in the ON (or cool) position (field-supplied pilot
relay energized) so that the zone thermostats control the boxes.
The field supplied fan operation and VAV box relay contacts
can be wired in series so that the boxes open when the uni
t is
not operational.
Units with One-Stage Heat
The VAV Box Output should be used to override zone
thermostat control and drive the VAV boxes fully open when
heating is required. While the unit is in Startup, Recirc, or any
heating operating state (UnocHtg, MWU, or Heating), the
VAV Box Output is in the OFF (or heat) position
(field-installed pilot relay de-energized).
VAV units have a “post heat” control feature that
forces the
discha
rge air volume to a minimum before closing the VAV
Box Output when the unit leaves the Recirc or any other
heating operating state. “Post heat” operation prevents
excessive duct static pressure conditions that could otherwise
occur when the zone thermostats regain VAV box control.
1
When the unit is not in Startup, Recirc, or any other heating
state and post heat operation is not active, the VAV Box Output
is in the ON (or cool) position (field-supplied pilot relay
energized) so that the zone thermostats control the boxes.
The field-supplied fan operation and VAV box relay contacts
can be wired in series so that the boxes open when the unit is
not operational.
The total VA of all field-mounted relays cannot exceed 15 VA,
and they must have a 24 VAC Class 2 coil. Other power can
cause unit malfunction.
1. The setting of a “post heat” timer determines the
duration of post heat operation. This timer is set to
zero at the factory and must be set to a non-zero
value to enable the “post heat” function. For more
information on post heat operation, refer to “Post
Heat Operation” in the “Discharge Fan Airflow
Control” section of the applicable VAV operation
manual (see T
able 1 on
page 1).
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