AAON VCCX2 User Manual

45VCCX2 Controller Technical Guide
SEQUENCE OF OPERATIONS
Modes of Operation
Variable Air Volume Operation with Supply Air
Tempering (Variable Air Volume Operation with
Outdoor Air Temperature Control)
On a VAV unit that may need daytime heating in order to maintain
the Cooling Supply Air Setpoint, previous controllers used a
supply air tempering sequence with the Supply Air Temperature
Sensor congured as the controlling sensor. The VCCX2 Controller
accomplishes the same result using the Hood-On Outdoor Air
Setpoints to initiate Cooling and Heating.
To utilize this sequence, the HVAC Mode Enable Source must be
congured as supply air tempering. Then, congure the Hood On
HVAC Setpoints for the outdoor air temperature values that will
enable Cooling and Heating. The Hood On Heating Setpoint should
be set at or above the outdoor air temperature, that when mixed with
the return air (with the economizer at its minimum position), will
require Heating in order to achieve the Heating Supply Air Setpoint.
The Hood On Cooling Setpoint would be set above that, which will
allow a Vent Mode in between.
Then, congure the Cooling and Heating Supply Air Setpoints.
While not set at the same value, those would normally both be set
at or near 55°F to allow the box heat to keep spaces comfortable.
With this conguration, as the outdoor air temperature rises above
the Hood On Cooling Setpoint, the unit will be in Cooling Mode,
controlling to the Cooling Supply Air Setpoint. The economizer
can operate as normal for free cooling to maintain the Cooling
Supply Air Setpoint.
During the Vent Mode, when the outdoor air temperature is between
the Hood On Cooling and Heating Setpoints, the economizer can
modulate if necessary to maintain the Cooling Supply Air Setpoint.
Whenever the outdoor air temperature falls below the Hood On
Heating Setpoint, the unit will be in Heating Mode controlling to
the Heating Supply Air Setpoint. The economizer can still modulate
during Heating Mode if the supply air temperature is too warm, and
it will control to a setpoint calculated to be 2°F above the Heating
Supply Air Setpoint. In this way, the economizer will have time to
close to its minimum position before the supply air temperature
falls below the Heating Supply Air Setpoint and Heating is initiated.
During Morning Warm-Up, heating will be controlled to the
Morning Warm-Up Supply Air Setpoint.
In this operation, if night setback operation will be initiated by
a Space Sensor connected to the VCCX2 Controller, then the
night setback Cooling and Heating osets will be applied to the
normal Occupied HVAC Mode Enable Setpoints (not the Hood
On Setpoints). During night setback operation, Heating will be
controlled to the Morning Warm-Up Supply Air Setpoint. Finally,
congure the VCCX2 for duct static pressure control.
Electronic Expansion Valve Operation
If using electronic expansion valves with VFD Compressors and
the RSMV Module, then a coil (suction line) temperature sensor will
measure the coil (suction line) temperature after each evaporator
coil line for each compressor, and this sensor will be connected to
an RSMV Module. This temperature will be used in conjunction
with the calculated saturated refrigerant temperature to calculate the
superheat of each evaporator coil. The electronic expansion valve for
each coil will then be controlled to maintain the Superheat Setpoint.
Head Pressure Control
The RSM can monitor a Head Pressure Transducer and control a
condenser fan to maintain a Head Pressure Setpoint.
In Cooling Mode, the condenser signal will modulate to maintain
the Cooling Head Pressure Setpoint.
In Dehumidication Mode, the condenser output signal controls to
the Reheat Head Pressure Setpoint. High head pressure conditions
produce the same eects as in the Cooling Mode.
See the appropriate RSM Technical Guide for a more detailed
sequence of operation.
Evaporative Condenser Operation
If the unit has been congured for evaporative condenser control and
the outdoor air temperature is above the Evaporative/Modulating
Condenser Low Ambient Setpoint, the evaporative condenser pump
relay will energize to be used as the rst stage of head pressure
control when the compressors are active. Modulating condenser
fan control will be the second stage.
If the outdoor air temperature is below the Evaporative/Modulating
Condenser Low Ambient Setpoint, the modulating condenser fan
will be used as the rst stage of head pressure control, and the
evaporative condenser pump relay will energize as the second stage.
If the head pressure is above the Cooling Head Pressure Setpoint by
the Evaporative Head Pressure Deadband Setpoint, the second stage
of head pressure control will be enabled. At this level below the
Cooling Head Pressure Setpoint, the second stage of head pressure
control will be disabled.
Sump Heater Operation
The sump heater relay activates if the sump temperature is below
the Sump Heater Enable Temperature Setpoint. The sump heater
relay deactivates once the sump temperature rises 10ºF above the
Sump Heater Enable Temperature Setpoint or if the sump drain
relay is active.

AAON VCCX2 Specifications

General

Brand	AAON
Model	VCCX2
Category	Controller
Language	English