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VCCX2 Controller Technical Guide
MUA Operation
Occupied Mode:
The VCCX2 will use the normal Cooling and
Heating Mode Enable Setpoints (not the Hood On Setpoints)
in conjunction with the Outdoor Air Temperature Sensor to
determine the mode of operation. The Outdoor Air Dewpoint
Setpoint will initiate the Dehumidification Mode. The Outdoor
Air Damper will be modulated to the Economizer Minimum
Damper Position (normally set at 100% for a MUA unit).
Unoccupied Mode:
Normally, an MUA unit is off during the
Unoccupied Mode. However, if the unit has Return Air, it can be
configured to operate as a recirculating Night Setback controlled
unit during unoccupied hours. This is accomplished by simply
configuring Night Setback Temperature Setpoints (anything other
than the default 30ºF) on a unit that is also configured for outdoor
temperature control (MUA). With this configuration, when the
unit goes Unoccupied, it will close the Outdoor Air Damper and
begin to use a Space Temperature Sensor in conjunction with
the existing Heating and Cooling Setpoints, offset by the Night
Setbacks, to make Night Setback calls. If a Space Humidity
Sensor is installed, and the unit is configured for Night Humidity
control, the VCCX2 Controller will use the Space Humidity
Setpoint for Unoccupied Dehumidification calls.
CAV/MUA Dual Mode (Hood On/Off
Operation)
The VCCX2 Controller can be configured as a CAV controller
but switch to MUA operation when an exhaust hood is energized.
This MUA Force Mode occurs when a 24 VAC wet contact
closure is received on the Hood On binary input on the VCCX2
Controller. Under normal operation (CAV), the unit will operate
as a recirculating Space Temperature (and Space Humidity)
controlled unit. This sequence should not be used on a VAV unit.
When the Hood On contact is made, the unit will open the
Outdoor Air Damper to its full open position. The Heating and
Cooling Modes will then be determined by the Outdoor Air
Temperature Sensor using the Hood On Outdoor Air Heating and
Cooling Setpoints which are used only in Hood On operation.
Dehumidification would then be initiated by an Outdoor
Dewpoint Setpoint. If using Reverse Building Pressure Control
using the Outdoor Air Damper, the Hood On Input will not affect
the damper position.
When the Hood On Force Mode is removed, the unit will revert
to CAV operation with the Outdoor Damper returning to its
minimum position (unless economizer operation is enabled)
and with mode control initiated by the Space Temperature and
Humidity Sensors.
Space Temperature Control of High
Percentage Outdoor Air Units
This option allows for Space Temperature control of 100%
outdoor air units or units with a high percentage of outdoor
air (normally 50% or greater). For this application you would
configure “Space Temperature w/High Percentage OA” for
the Controlling Sensor option. The intent of this sequence is
to allow Space Temperature and Humidity control of the unit
while preventing the dumping of hot or cold outside air into the
space during the Space Vent Mode of operation. If a Return Air
Humidity Sensor is available, it can be configured to initiate
Dehumidification.
As long as there is a Space Temperature call for Cooling or
Heating or if there is a Space Dehumidification call, the unit will
remain under Space Control. During this Space Control operation,
if the configured minimum Outdoor Air Damper (Economizer)
position is less than 100%, the Economizer can open farther
for free cooling during Space Cooling Mode. Additionally, the
Economizer minimum position can be overridden by IAQ CO
2
conditions.
If both the Space Temperature and the Space Humidity are
satisfied, before switching to the Space Vent Mode, the controller
compares the Outdoor Air Temperature to the Hood On HVAC
Setpoints (Hood On MUA Setpoints) to determine if a continued
demand for heating or cooling is required to prevent dumping.
If there is no demand, the VCCX2 Controller switches to Vent
Mode. If the Outdoor Air Temperature is greater than the Hood
On HVAC Cooling Setpoint or less than the Hood On HVAC
Heating Setpoint, plus the Occupied Deadband, the VCCX2
Controller will continue mechanical cooling or heating operation
and stage or modulate it as necessary to maintain the Vent Mode
Supply Air Setpoint (Calculated to be halfway between the
Space Heating and Cooling Mode Enable Setpoints). During this
outdoor control, if the configured minimum Outdoor Air Damper
(Economizer) position is less than 100%, the minimum position
can be overridden by IAQ CO
2
conditions.
While the unit is under Outdoor Air Temperature control,
Dehumidification Mode is then initiated based on an Outdoor
Air Dewpoint Setpoint with reheat controlling to the Vent Mode
Supply Air Setpoint.
Any call for space Cooling, Heating, or Dehumidification will
cancel the outdoor operation and return the unit to Space Control.
NOTE:
All Minimum Run times must be satisfied before
mechanical cooling or heating is de-energized.
SEQUENCE OF OPERATIONS
MUA, CAV/MUA, and Space Temperature Control
