41VCCX-454 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 oset.
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 satised.
Modulating Heating
The VCCX-454 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 VCCX-454 Controller with a user-adjustable voltage
range between 0-10 VDC. Modulating gas, which is controlled
by a MODGAS-X series module, is not included in this section.
Whichever form of modulating heating is used, the VCCX-454
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 dierential
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 VCCX-454 supports the use of a direct re heater. The VCCX-
454 sends and receives signals from a direct re heater controller
made by another manufacturer. The modulating heating signal
(AO3) on the VCCX-454 controller is wired to the direct re
controller to adjust the amount of heat it puts out. The VCCX-454
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
EM1 module. The VCCX-454 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 VCCX-454 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 EM1 module are used to monitor
Direct Fire Status and Alarms from the direct re controller. If the
VCCX-454 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 VCCX-454 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 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.
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