System Management / Field Installed
Demand Controlled Ventilation / ComfortID
Demand Controlled Ventilation / Sequence of Operations
Note: The following is a sample of a typical sequence of operation that may be adapted for a
particular project. This is provided as a suggestion on how to write a sequence of operation for a
specific project. Consult your Carrier representative if the sample sequence requires alterations.
4.3.4 Demand Controlled Ventilation
A. Overview:
The HVAC system sequence of operation steps for normal thermal control is unchanged by
incorporation of DCV. The CO2 sensors represent a third function that may override the normal
thermal controls in some situations. The sequences included below discuss the ventilation functions
only and must be added to the design’s description of thermal controls to create complete sequences
of operation.
B. Constant-Volume — Start-up Purge Cycle:
1. When the AHU starts, the outdoor air damper shall open, initiating a timed purge cycle. The out
door air damper shall modulate to maintain the mixed airflow at ______ percent outdoor air. The
outdoor air percentage shall be calculated as the following ratio: %OA = (MATº - RATº) / (OATº RATº). The purge period shall be adjustable and shall initially be set for ____minutes.
2. The AHU shall modulate its preheat control to maintain the discharge air temperature setpoint if the
mixed air temperature falls below the AHU discharge air temperature setpoint. At the conclusion of
the timed cycle, the outdoor air damper shall modulate closed to maintain the base ventilation rate
of _____ percent outdoor air, and the demand controlled ventilation control algorithm shall be
enabled.
C. Constant-Volume — Single-Zone and Multi-Zone Applications:
1. All zones served by a given air-handling unit (AHU) shall be polled and the highest CO2 sensor
reading shall be sent to the unit (outside air damper) controller. This CO2 reading shall be
compared to the CO2 setpoint at the system controller. If the reading is below the CO2 setpoint,
the AHU shall maintain the base ventilation rate at ____ percent outdoor air. If the reading is
above the setpoint, the outdoor air damper controller shall modulate the dampers open utilizing a
proportional-integral (PI) loop to reduce the CO2 concentration in the space.
2. The AHU shall modulate its preheat control to maintain the discharge air temperature setpoint if the
mixed air temperature falls below the AHU discharge air temperature setpoint. Once the space
CO2 level drops below the CO2 setpoint, the outside air dampers shall modulate to maintain the
base ventilation rate.
3. If the AHU is equipped with economizer control and the economizer determines that it is beneficial
to use outside air for cooling, the economizer shall override the demand controlled ventilation
algorithm to modulate the dampers open to their maximum position.
D. Variable-Air-Volume DCV Systems:
1. Each variable-air-volume zone controller shall monitor primary air flow, space temperature, air
handler status and mode, supply air temperature (as applicable) and shall position its terminal
damper based on its proportional-integral-derivative (PID) temperature control algorithm to
maintain the desired zone temperature setpoint. Each zone controller shall include the inherent
ability to override the temperature control loop and modulate the terminal’s damper with a
proportional-integral (PI) loop, based on a CO2 sensor with its associated setpoint schedule, in
conjunction with the normal temperature control loop. The zone controller shall be capable of
maintaining a ventilation setpoint through a demand controlled ventilation (DCV) algorithm in
conjunction with the air-handling unit to fulfill the requirements of ASHRAE standard, 62-1989
“Ventilation For Acceptable Indoor Air Quality” (including Addendum 62a-1990).
2. The DCV control function shall determine the zone ventilation airflow based on the CO2 zone
sensor input signal. When the DCV function is enabled, the zone controller shall override
(increase) the primary airflow in order to provide additional ventilation if the airflow is insufficient to
meet the zone CO2 setpoint. The control algorithm shall use a proportional-integral (PI) algorithm
to determine the required airflow in order to prevent the CO2 sensor reading from exceeding the
desired zone setpoint.
3. Whenever the system air-handling unit (AHU) is operating, the system controller shall maintain the
base ventilation rate (minimum ventilation rate) unless overridden by a pre-occupancy purge
sequence or the DCV function.
4. All zone controllers working with a given AHU shall be polled and the highest CO2 sensor reading
shall be sent to the system controller. This CO2 reading shall be compared to the system CO2
setpoint. If the reading is below the system CO2 setpoint, the AHU shall maintain the base
ventilation rate. If the reading is above the setpoint, the AHU damper controller shall modulate the
outside air dampers open using a proportional-integral (PI) loop to maintain the zone CO2 at the
zone setpoint.
5. The system controller shall modulate the AHU preheat control to maintain the discharge air
temperature setpoint if the mixed air temperature falls below the discharge air temperature
setpoint. The outside air damper position shall close with a decreasing CO2 sensor signal down to
the base ventilation rate. The zone controller shall contain a provision to operate modulating type
heat to maintain the space temperature at the midpoint between the heating and cooling setpoints
during DCV operation. The zone controller shall have the capability to define a maximum primary
airflow limit (ventilation) to protect the zone from overcooling for those units that do not include
local heating. DCV control shall be automatically suspended if the space temperature falls below
the heating setpoint and the outdoor air damper shall return to the base ventilation position.
6. If the AHU is equipped with economizer control and the economizer determines that it is beneficial
to use outside air for cooling, the economizer shall override the DCV algorithm to modulate the
dampers open to their maximum position.
7. Operation shall be dependent upon the equipment mode of operation, so that the DCV function will
only operate during occupied periods when the outdoor air damper is actively providing ventilation.
DCV control shall be disabled if the CO2 sensor fails.
E. Product Integrated Controllers:
The unit controller shall have the capability to provide demand controlled Ventilation (DCV) function
using the input signal from a CO2 sensor located in the conditioned space. Control functions shall
include outdoor air damper modulation, monitoring, and alarm generation.
The unit controller shall maintain an adjustable CO2 setpoint by overriding the mixed-air damper
position and modulating the damper further open to provide the required ventilation. The DCV
algorithm shall automatically limit the amount of outdoor air to prevent the mixed-air temperature from
falling below 50ºF.
The unit controller shall also have the ability to limit the maximum amount of outdoor air during DCV
operation.
If heating is available during DCV operation, the heating valve [face and bypass damper] shall
modulate to maintain a minimum supply air temperature of 65ºF, if neither heating nor cooling is
required by the space.
If the space temperature exceeds an adjustable high limit value or falls below an adjustable low limit
value, or if the space humidity exceeds the humidity setpoint, the algorithm shall disable any DCV
damper override and maintain the normal minimum ventilation setpoint or to the configured mixed-air
temperature setpoint until the space temperature and/or space relative humidity returns to normal.