Vav Box Output - McQuay MicroTech II Installation And Maintenance Manual

to turn off, blink on and off rapidly, blink on and off slowly,
or remain on (no alarm indication). For details regarding
how to use the keypad to configure these alarms, refer to the
"Alarm Monitoring" section of the applicable operation
manual (Refer to Table 1 on page 3).

VAV Box Output

The VAV Box Output (MCB-B012) supplies 24 VAC to ter-
minal 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 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 "Output Schematic: Actuator
Control" on page 36 or "Output Schematic: Auxiliary VFD
Control" on page 37.
CAUTION
The total VA of all field-mounted relays must
exceed 15 VA and they must have a 24 VAC
Class 2 coil.
The VAV Box Output (MCB-BO12) is designed to coordi-
nate unit operation with VAV box control. Field use of this
output is optional; however, it is highly recommended, espe-
cially for VAV systems that have heating capability (unit or
duct mounted).
The following are application guidelines for four basic heat-
ing 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 tem-
peratures.
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 oper-
ating 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 which forces
the discharge air volume to a minimum before turning on the
VAV Box Output when the Recirc operating state is com-
plete. Post heat operation prevents excessive duct static pres-
sure that could otherwise occur when the zone thermostats
1
regain VAV box control.
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.
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 (Refer to Figure 1 on page 5).
14
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.
Cooling Only Units with Field Supplied Heat
For VAV systems with cooling only units and duct mounted
reheat coils, the VAV Box Output can override zone thermo-
stat control and drive the VAV boxes fully open when heat-
ing 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 which forces
the discharge 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
1
VAV box control.
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 unit is
not operational.
Units with Staged Heat
The VAV Box Output should be used to override zone ther-
mostat 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 which forces
the discharge 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 other-
wise occur when the zone thermostats regain VAV box con-
1
trol.
When the unit is not in Startup, Recirc, or any other heating
state and post heat operation is not active, the VAV Box Out-
put 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.
IM710