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142 RT-SVD03L-EN
Demand Control Ventilation (DCV)
C0
2
Sensor Connections
Demand Controlled Ventilation (DCV) describes a
control strategy that responds to the actual demand
(need) for ventilation by regulating the rate at which
the HVAC system brings outdoor air into the building.
There are several ways to assess ventilation demand:
• Occupancy sensors, which detect the presence or
number of people in each monitored space.
• Occupancy schedules, which allow a building
automation system to predict the current
population based on the time of day.
• Carbon dioxide (CO
2
) sensors, which monitor the
concentration of CO
2
that the occupants
continuously produce.
Regardless of which method is used, DCV strategies
vary the outdoor air intake in response to the current
population. The practice of using carbon dioxide
concentration as an indicator of population or
ventilation rate is often called CO
2
-Based, Demand-
Controlled Ventilation.
The CO
2
DCV function is only available for units with
economizers; the 50% motorized damper option will
not operate with a CO
2
sensor. The 50% motorized
damper opens to one position; it does not modulate
like the economizer. Also, the 50% damper does not
come with a return blade or a mixed air sensor.
The CO
2
option does not require an RTOM (3-25 tons)
board.
The CO
2
sensor can be configured for 0-10 vdc, 0-20
ma, or 4-20 mA analog outputs. For use with the
ReliaTel
™ economizer, the sensor must be set for 0-10
vdc. As the CO
2
level increases, the voltage output
increases accordingly.
The potentiometers used for setting the CO
2
setpoints
and outdoor air damper position setpoints are located
on the ReliaTel
™ RTEM module.
Table 99. CO
2
levels and associated voltage outputs
CO2 Level (ppm)
Voltage Output
(vdc)
0 0
200 1
400 2
600 3
800 4
1000 5
1200 6
1400 7
1600 8
Table 99. CO
2
levels and associated voltage outputs
(continued)
CO2 Level (ppm)
Voltage Output
(vdc)
1800 9
2000 10
RTEM Operation
Units equipped with an RTEM economizer logic
module will perform Demand Controlled Ventilation
differently based on the RTRM version also installed in
the unit. See below for information concerning the
different configurations of RTEM and RTRM versions.
RTEM w/ RTRM v8.0 and Later
For units equipped with an RTRM v8.0 or later and also
an RTEM, the control will utilize two separate Space
CO
2
Setpoints and two separate Damper Minimum
Position Setpoints as described below:
CO
2
Setpoints
The CO
2
setpoints will be obtained through two
onboard potentiometers located on the RTEM; Building
Design CO
2
Setpoint (upper limit) and DCV minimum
CO
2
setpoint (lower limit). The upper limit CO
2
setpoint
will have a range of 1000-2000 ppm and the lower limit
CO
2
setpoint will have a range of 300-1900 ppm. A 100
ppm differential will be enforced between the upper
limit CO
2
setpoint and lower limit CO
2
setpoint. In the
case of the lower limit CO
2
setpoint being set to
compromise this 100 ppm differential, the upper limit
CO
2
setpoint will not be "pushed-" and the 100 ppm
differential will be enforced. However, if the upper limit
CO
2
setpoint is set to compromise the 100 ppm
differential, the lower limit CO
2
setpoint will be pushed
down in order to enforce the 100 ppm differential and
to allow the upper limit CO
2
setpoint to be set as
desired.
OA Damper Min Position Setpoints
The OA damper min position setpoints will be
determined by the position of two onboard
potentiometers located on the RTEM; Building Design
Min Position Setpoint (10-50%) and DCV Min Position
Setpoint (0-40%). A 10% differential will be enforced
between the design min position setpoint and DCV min
position setpoint; the DCV min position setpoint will
always be 10% less than the design min position
setpoint. If the unit is configured for DCV and a remote
min position value is present on the P0 and P1
terminals, the remote min position will become the
design min position setpoint and the 10% differential
will not be enforced. If the remote min position is set to
be lower than the DCV min position setpoint, the