AAON VCCX2 User Manual

33VCCX2 Controller Technical Guide
SEQUENCE OF OPERATIONS
Modes of Operation
Heating Mode
Available heating options are staged gas, modulating gas, staged
electric, on/o hot water, modulating hot water, and modulating
SCR electric.
Occupied Heating is enabled when the temperature at the Mode
Enable Sensor falls one deadband below the Heating Setpoint.
Heating is disabled when the Mode Enable temperature raises one
deadband above the Heating Setpoint.
Unoccupied Heating operation is enabled when the space
temperature falls below the Heating Mode Enable Setpoint minus
the unoccupied heating oset.
In the Heating Mode, as the supply air temperature falls below the
active Supply Air Heating Setpoint, the heating will begin to stage
on or to modulate. Each stage must meet its minimum o time
(adjustable) before it is allowed to energize, and successive stages
are subject to a heating stage up delay (adjustable).
Heating stages will continue to run until the supply air temperature
rises above the active Supply Air Temperature Setpoint plus the
heating stage control window at which point the heating will begin
to stage o. Each stage must meet its minimum run time (adjustable)
before it is allowed to stage o, and successive stages are subject
to a heating stage down delay (adjustable).
Mechanical heating is disabled if the outdoor air temperature rises
1°F above the Heating Lockout Setpoint and will remain disabled
until the outside air temperature falls 1°F below the Heating Lockout
Setpoint. If the outside air temperature disables mechanical heating
while it is currently operating, mechanical heating will stage o as
minimum run times and stage down delays are satised.
Modulating Heating
The VCCX2 supports various forms of modulating heat such as
SCR electric heat, modulating hot water heat, and modulating steam
heat. This references modulating heat that is controlled from AO3
on the VCCX2 Controller with a user-adjustable voltage range
between 0-10 VDC. Modulating gas, which is controlled by the
MODGAS-X Module, is not included in this section. Whichever
form of modulating heating is used, the VCCX2 will modulate the
heat source to achieve the active Supply Air Temperature Setpoint.
The modulating heating proportional window is used to determine
the signal to the modulating heating source and is user-adjustable.
The modulating heating signal is calculated by the dierential
between the supply air temperature and the active Supply Air
Temperature Setpoint based on the modulating heating proportional
window. The maximum signal adjustment per time period is 10%
and is not user-adjustable.
The minimum signal adjustment per time period is based on the
modulating heating proportional window. The larger the modulating
heating proportional window, the smaller the signal adjustment will
be per time period. The time period is the delay between another
increase or decrease in the modulating heating source signal and
is user-adjustable.
For example, if the modulating heating proportional window is 5°F,
the signal will be adjusted 2% per 1°F each time period above or
below the active Supply Air Temperature Setpoint. When the supply
air temperature is above or below the active Supply Air Temperature
Setpoint by 5°F or more, the signal will adjust 10% each time period.
Direct Fire Heating
The VCCX2 supports the use of a direct re heater. The VCCX2
sends and receives signals from a direct re heater controller made
by another manufacturer. The modulating heating signal (AO3) on
the VCCX2 controller is wired to the direct re controller to adjust
the amount of heat it puts out. The VCCX2 uses the same modulating
heating setpoints that are used for other types of heat
When a direct re heater is installed on a non-MUA unit it requires
the addition of a mixed air sensor wired to the T2 input on the
VCCX2 EM1 module. The VCCX2 uses the mixed air sensor and
the supply air sensor to determine the heat rise the direct re heater
is providing. The max heat rise limit is calculated based on the
outdoor air damper position. If necessary, the VCCX2 will limit
the supply air setpoint it is trying to reach to prevent the heat rise
from exceeding the max heat rise limit.
A 0-5000ppm CO
2
located in the return duct is also required on
non-MUA units. If the measured return CO
2
rises above the Direct
Fire CO
2
Max Level setpoint the direct re heater will be disabled
until the CO
2
drops below the CO
2
Accepted Level setpoint. If the
max level is exceeded 3 times in one hour the direct re heat will
be locked out.
Binary inputs 2 and 3 on the VCCX2 EM1 module are used to
monitor Direct Fire Status and Alarms from the direct re controller.
If the VCCX2 doesn’t receive a Direct Fire Status signal within 10
minutes of enabling the direct re heater it will remove the call for
heat and lockout the direct re heater. It will also generate a lockout
if a Direct Fire Alarm signal is active.
Before activating the heater, the VCCX2 will go through a purge
mode to verify the OA damper is moving. It will start opening the
damper until it sees the mid-switch signal on the VCCX2’s EM1
SIG 2 input switch state. Once it determines the damper is good it
will put the damper at the OA Damper Position % setpoint value.
If the damper is determined to be bad the direct re heating will
be locked out.

AAON VCCX2 Specifications

General

Brand	AAON
Model	VCCX2
Category	Controller
Language	English