AAON VCCX2 User Manual

44VCCX2 Controller Technical Guide
SEQUENCE OF OPERATIONS
Modes of Operation
During this space control operation, if the congured 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 indoor air quality CO
2
conditions.
If both the space temperature and the space humidity are satised,
before switching to the Space Vent Mode, the controller compares
the outdoor air temperature to the Hood On HVAC Setpoints (Hood
On Make-up Air Setpoints) to determine if a continued demand for
heating or cooling is required to prevent dumping.
If there is no demand, the VCCX2 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 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 congured minimum
outdoor air damper (economizer) position is less than 100%, the
minimum position can be overridden by indoor air quality CO
2
conditions.
While the unit is under outdoor air temperature control,
Dehumidication Mode is initiated by the Outdoor Air Dewpoint
Setpoint with reheat controlling to the Vent Mode Supply Air
Setpoint. Any call for space Cooling, Heating, or Dehumidication
will cancel the outdoor operation and return the unit to space control.
NOTE: All minimum run times must be satised before
mechanical cooling or heating is de-energized.
Single Zone Variable Air Volume
Single zone VAV is an application where the supply fan VFD
modulates to maintain the Space Temperature Setpoint while heating
or cooling is modulated to maintain the Supply Air Setpoint. This
application can be congured to use VAV Cooling and either VAV
Heating or CAV Heating. There is no Supply Air Temperature
Setpoint reset function on a single zone VAV unit.
VAV Cooling and VAV Heating require modulating cooling
and heating sources in order to maintain a constant supply air
temperature no matter what the fan speed is. CAV Heating must be
congured if using a staged form of heat.
The Space Temperature Sensor (cannot use return air temperature)
determines the heating or cooling mode of operation.
In the Cooling Mode, the modulating cooling source will modulate
to maintain the Cooling Supply Air Setpoint. The supply fan VFD
will begin operation at a user-adjustable minimum VFD cooling
speed (30% default) and modulate between this setpoint and 100%
as needed to maintain the space temperature within the space cooling
reset window created by conguring a Space Cooling High and a
Space Cooling Low Reset Source Setpoint.
If the unit is congured for variable air volume heating, then in
the Heating Mode the modulating heating source will modulate
to maintain the Heating Supply Air Setpoint. The supply fan VFD
will begin operation at a user-adjustable minimum VFD heating
speed (50% default) and modulate between this setpoint and the
maximum VFD heating speed (100% default) as needed to maintain
the space temperature within the space heating reset window created
by conguring a Space Heating High and a Space Heating Low
Reset Source Setpoint.
If the unit needs to be congured for CAV heating, set the minimum
VFD heating speed to be the same as the maximum VFD heating
speed desired during heating. Once the unit enters the Heating
Mode, the supply fan will run at the set maximum VFD heating
speed (100% default).
In the Vent Mode of operation, the supply fan will operate at the VFD
vent speed (user-adjustable).During Dehumidication, the fan will
operate as described above, depending on if the space temperature is
calling for Cooling Dehumidication or Heating Dehumidication.
When the controller is in the Vent-Dehumidication Mode, the
supply fan VFD will still modulate based on the space temperature.
It looks at a temperature half way between the Cooling and Heating
Setpoints and modulates between the Cooling minimum and 100%
as the space temperature goes from the midway value to 0.5°F
above that value.
Night setback operation uses the same single zone VAV fan control
logic. However, since the setbacks would normally be larger than
the space reset range for the fan speed, the fan will typically always
be at 100%.
If the hood on/o operation is used on a single zone VAV unit, then
during hood on, the mode enable will switch to the Outdoor Air
Temperature Sensor using Hood On Mode Enable Setpoints and the
outdoor air damper will modulate to 100%. The supply fan VFD
will modulate to 100%.
SZVAV Control of High Percentage Outdoor Air Units
This option allows for SZVAV control of units with a high percentage
of outdoor air (normally 50% or greater). For this application
congure “SZVAV w/High OA Content” 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.
This conguration uses the same sequence described in the Space
Temperature Control of High Percentage Outdoor Air Units section
for determining modes of operation. When in Space Vent Mode
the supply fan will operate at the minimum VFD percentage for
the mode of operation being calculated by the outdoor conditions.

AAON VCCX2 Specifications

General

Brand	AAON
Model	VCCX2
Category	Controller
Language	English