81
VCCX2 Controller Technical Guide
Airflow Monitoring
Outdoor, Supply, Return and Exhaust Airflow can be monitored
using the EBTRON
®
GTC116 or HTN104 series, Paragon
MicroTrans
EQ
series, or GreenTrol
™
GA-200-N Module in
conjunction with a GreenTrol
™
GF series of airflow station.
Contact AAON Controls for information on other airflow station
options. The VCCX2 will control the Outdoor Air Damper to
maintain an Outdoor Air CFM Setpoint. This operation can be
overridden higher by normal Economizer control.
On an MUA unit or a unit configured for Space Control of High
Percentage Outdoor Air, since the damper is typically at 100%,
the VCCX2 can be configured to modulate the Supply Fan VFD
to maintain an Outdoor CFM Setpoint.
Preheater Operation
A Preheat Relay can be configured to energize anytime the Supply
Fan is operating and the Outdoor Air Temperature is below the
Preheat Setpoint. This option allows preheating of cold outside
air before it reaches the evaporator coils and is useful in Hot
Water/Chilled Water applications or during CO
2
control of the
economizer in low temperature conditions. This operation only
occurs in the Occupied Mode.
If using the PREHEAT-X, an SCR Preheater and/or stages of
Preheat can be controlled. If the Entering Air Temperature (sensor
connected to the PREHEAT-X) falls below the Preheat Setpoint,
then Preheat will be controlled to either a Cooling, Heating or
Vent Mode Preheater Leaving Air Setpoint - depending on if
the VCCX2 is currently in the Cooling, Heating, or Vent Mode.
These setpoints are all set in the VCCX2 Controller. If using
the PREHEAT-X in conjunction with the VCCX2 Controller,
the Entering Air Temperature on the PREHEAT-X (instead of
the Outdoor Air Temperature Sensor connected directly to the
VCCX2 Controller) will be used as the temperature that locks
out compressors. See the
PREHEAT-X Controller Technical
Guide
for more details.
Low Ambient Operation
A Low Ambient Relay can be configured. Whenever the Outdoor
Air Temperature falls below the Low Ambient Setpoint, this Low
Ambient Relay will energize. This operation occurs in both the
Occupied and Unoccupied Modes of Operation.
Heat Wheel
One of the relay outputs can be configured as a Heat Wheel Relay.
This relay will enable the Heat Wheel when the unit goes into
the Occupied Mode. If the unit is a recirculating unit configured
to use Economizer free cooling, the relay will disable during
Economizer operation.
If the unit is a 100% outdoor air unit configured to use the Outdoor
Air Sensor as the controlling sensor, then the Heat Wheel Relay
will disable in between the Outdoor Air Cooling and Heating
Mode Enable Setpoints (Outdoor Air Vent Mode). Similarly, on
a unit configured for Space Control of High Percentage Outdoor
Air, and the unit is in Outdoor Air Vent Mode, the wheel is
disabled. The controller can also be configured to disable the
Heat Wheel Relay between High and Low Outdoor Air Enthalpy
Setpoints.
Heat Wheel Defrost
A Defrost Mode will occur if the Heat Wheel relay is active, the
Outdoor Temperature is below the Heat Wheel Defrost Setpoint,
and 30 minutes have elapsed since the last Defrost Mode. The
Defrost Mode will disable the Heat Wheel Relay for 2 minutes.
Before the Heat Wheel Relay disables, a 2-minute delay will
occur while the Economizer closes to a 10% position to allow
the return air to have maximum defrost effect on the wheel. At
the end of the Defrost Mode, the Heat Wheel Relay will enable,
and the economizer will return to its normal position. This 10%
re-positioning will not occur on units configured for MUA control
(outdoor air is the controlling sensor), or for units in Hood-On
operation, or for units that are configured for Space Temperature
Control of a high percentage of outdoor air.
Duct Static Pressure Control
If the VCCX2 Controller has been configured for Duct Static
Pressure Control, then anytime the Supply Fan is operating, the
unit will be controlling to a Duct Static Pressure Setpoint. The
Static Pressure Control Output Signal can be used to control
a Supply Fan VFD (Direct Acting Operation), or a Zoning or
Bypass Damper Actuator (Reverse Acting Operation).
The Duct Static Pressure Setpoint, the Setpoint Deadband, the
Static Pressure Control Signal, and the Static Control Rate are
all user-adjustable. The Static Control Rate is the amount of time
that elapses between each adjustment to the Duct Static Pressure
Control Output Signal. The default period is 10 seconds and
should not be changed unless close observation reveals that the
Supply Fan or Bypass Damper is hunting and not maintaining a
stable pressure reading.
For Supply Fan VFD operation, the Output Signal increases
(increases the VFD speed) if the Duct Static Pressure is below
the Duct Static Pressure Setpoint by the Deadband amount, and
the Output Signal decreases (decreases VFD Speed) if the Static
Pressure is above the Setpoint by the Deadband amount.
SEQUENCE OF OPERATIONS
Airflow Monitoring, Preheater, Low Ambient and Duct Static Press
