Communications wiring
Unit Ventilator
CARRIER CORPORATION ©2019
Integration Guide
All rights reserved
23
Changeover mode detection
The Unit Vent determines the changeover mode for 2-pipe heating/cooling systems. The controller monitors a
local changeover thermistor sensor or switch, dependent upon the configuration. For thermistor applications, the
heat or cool mode is determined by user-configurable temperature setpoints. When the sensed temperature
exceeds the
Changeover Heat Limit
, the system changeover mode is set to heat. When the sensed temperature
falls below the
Changeover Cool Limit
, the system changeover mode is set to cool. For applications using a switch,
the heat mode is determined when the input is open, while a closed switch indicates cool mode.
Additionally, an Analog Network Input point and a BACnet Analog Value input variable are also provided to allow a
network-supplied analog value of the system water temperature to be used to determine the changeover mode.
The Analog Network Input point has the highest priority, followed by the BACnet AV point, then the local input if
multiple inputs are supplied simultaneously.
Indoor Air Quality
If used with a modulating-type mixed air damper, the controller can provide
Demand Controlled Ventilation
(DCV)
to properly ventilate an occupied space. To meet ventilation requirement, the fan must be configured for the
Continuous
or
Always On
mode of operation.
Demand Control Ventilation (DCV)
- If the optional CO
2
sensor (or a CO
2
sensor value) is available and a
modulating mixed air damper is installed, the controller will increase the minimum damper opening to outdoor air
as required by the CO
2
levels within the occupied space. The control provides DCV during occupied periods. The
control monitors the CO
2
level and compares it to the configured setpoints and adjusts the minimum damper
position as required. The control provides proportional ventilation to meet the requirements of ASHRAE
specifications by providing a base ventilation rate and then increasing the rate as the CO
2
level increases. The
control begins to proportionally increase ventilation when the CO
2
level rises above the start ventilation setpoint
and reaches the full ventilation rate when the CO
2
level is at or above the maximum setpoint.
The configurable
Minimum Damper Pos
insures that proper base ventilation is delivered when occupants are not
present. The
DCV Max Vent Damper Pos
(default 60%) limits the maximum amount of outdoor air. An automatic
minimum mixed air temperature override control prevents unacceptable mixed air temperatures when the outdoor
air is extremely cold. It proportionally resets the maximum amount of outdoor air used for DCV from the configured
DCV Max Vent Damper Pos
as the OA drops below the
Minimum Cooling SAT
limit and ends at the
Minimum
Damper Pos
when the OAT reaches 10°F (-12.2°C).
If this additional outdoor air being introduced for ventilation causes an unacceptable drop in the supply air
temperature or could cause a potential coil freeze-up condition, then the control can be set to temper the supply
air during DCV control. The control uses heating to prevent the supply air from falling below the configurable
Min
DCV Override
SAT setpoint, when the DCV control is active. The IAQ configurations are accessed on the
Properties
page >
Equipment
tab >
Configuration
.
The following conditions must be true for this algorithm to run:
Damper Control
is configured for
Modulating
Valid space temperature must be available
The unit is in an occupied mode
Supply Fan
must be on and not in alarm
IAQ sensor value reading is greater than the
DCV Start Control Setpoint
The DCV calculated damper position must be > the economizer position
