Vav Box Output; Cooling-Only Units; Cooling-Only Units With Field Supplied Heat; Units With Staged Heat - McQuay MicroTech II Installation And Maintenance Manual

how to use the keypad to configure these alarms, refer to the
"Alarm Monitoring" section of the applicable operation
manual (see Table 1 on page 1).

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, wire the coil of a
field-supplied and installed 24 VAC pilot relay 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 30 or
"Output Schematic: Auxiliary AFD Control" on page 31.
The total VA of all field-mounted relays cannot exceed 15
VA and they must have a 24 VAC Class 2 coil. Improper
current can cause improper operation and equipment
damage.
The VAV Box Output (MCB-BO12) is designed to
coordinate unit operation with VAV 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).
Below are application guidelines for four basic heating
configurations. For all of these configurations, the VAV Box
Output (MCB-BO12) is off for an adjustable time period
after unit startup (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 volume 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 occur otherwise when the zone
thermostats regain VAV box control.
1. The setting of a "post heat" timer determines the duration of post
heat operation. This timer is set to zero at the factory. To enable
the "post heat" function set it to a non-zero value. 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 Table 1 on page 1).
McQuay IM 710-1
CAUTION
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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 the zone
thermostats control the VAV boxes.
Wire the field-supplied fan operation and VAV box relay
contacts in series so 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
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
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
occur when the zone thermostats regain 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.
Wire the field-supplied fan operation and VAV box relay
contacts in series so the boxes open when the unit is not
operational.

Units With Staged Heat

Use the VAV Box Output 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, 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
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 the zone thermostats control the boxes.
Wire the field-supplied fan operation and VAV box relay
contacts in series so the boxes open when the unit is not
operational.
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