AAON VCCX2 User Manual

38VCCX2 Controller Technical Guide
SEQUENCE OF OPERATIONS
Modes of Operation
O󰀨 Mode
O Mode occurs in the Unoccupied Mode when there is no heating
or cooling demand. The supply fan is o and the outside air damper
is closed.
O Mode can only occur in the Occupied Mode if the fan is
congured to cycle with heating and cooling and there is no call
for heating or cooling.
Economizer Operation
Economizer Operation (Standard )
The Economizer can operate as a sensible economizer if used with
an outside air temperature sensor or as an enthalpy economizer with
an E-BUS outdoor air temperature/humidity sensor.
Economizer operation is enabled when the outdoor air drybulb,
wetbulb, or dewpoint temperature falls below the Economizer
Enable Setpoint by 1°F and if the outdoor air temperature is at least
5°F below the return air temperature (if that value is available).
Economizer operation is disabled when the outdoor air temperature
rises 1°F above the Economizer Enable Setpoint.
The economizer acts as the rst stage of cooling and controls to
the active Supply Air Cooling Setpoint. An economizer minimum
position can be programmed into the controller. During economizer
operation, the economizer will modulate between this minimum
position and 100%. If the economizer reaches 100% for two minutes
and the supply air temperature is still above setpoint, mechanical
cooling is then allowed to stage up while the economizer is held
at the full open position. Any time cooling stages are currently
running, and the economizer becomes enabled, it will immediately
open to 100%.
During Heat and Vent Modes, the economizer will remain at its
minimum position. The only exception to this can occur during VAV
operation with outdoor air temperature control. During Unoccupied
Mode, the economizer can be used for night setback free cooling;
otherwise it will remain closed.
Indoor air quality (CO
2
) override of the economizer simply resets
the economizer minimum position higher.
If utilizing the Title 24 economizer option, an economizer feedback
signal (0-10 VDC) can be wired into the VCC-X EM1 Expansion
Module for status monitoring. Several Title 24 alarm conditions
can also be displayed.
Economizer Override Via BACnet
As stated above, the economizer must reach and remain at 100%
before compressors will be allowed to stage on to meet the Cooling
Supply Air Setpoint. Because of this 100% requirement, if a value
of less than 100% is written to BACnet point AV:70 (Outdoor Air
Damper Override) while compressors are operating, the controller
eectively recognizes this as the economizer being active at less
than 100% – so the compressor(s) will disable.
To avoid the economizer being disabled but compressors are
enabled, AV:49 (Minimum Economizer Position) can be used. This
will change the minimum position, while allowing compressors to
remain on. However, if using AV:49 and the economizer operation
is enabled, the damper can open more than the minimum position
for true economizer operation.
Comparative Enthalpy Economizer Operation
A comparative enthalpy economizer option is also available. The
E-BUS Outdoor Temperature/Humidity Sensor and the E-BUS
Return Air Temperature/Humidity Sensor must be used for this
operation to be available.
If the outdoor enthalpy is below the Comparative Economizer
Enable Setpoint by the comparative economizer enable deadband
amount, and the outdoor enthalpy is less than the return air enthalpy
by the comparative economizer enable deadband amount, then
economizer operation will be enabled to act as the rst stage of
Cooling.
If the outdoor air enthalpy rises above the Comparative Economizer
Enable Setpoint by the comparative economizer enable deadband
amount, or if the outdoor air enthalpy rises above the return air
enthalpy by the comparative economizer enable deadband amount,
then economizer operation will be disabled.
Comparative enthalpy economizer with dry bulb limit is also an
option. It combines the above comparative enthalpy operation
with enabling the economizer using the outdoor air dry bulb. Both
methods must be true for the economizer to be enabled. If either
method is not true, the economizer operation will be disabled.

AAON VCCX2 Specifications

General

Brand	AAON
Model	VCCX2
Category	Controller
Language	English