NJ GREEN BUILDING MANUAL
NEW COMMERCIAL UPDATED 5-MAY -11
Demand Control Ventilation
What is Demand Control Ventilation?
Demand-controlled ventilation (DCV) using carbon dioxide (CO 2) sensing is a
combination of two technologies: CO 2 sensors that monitor CO2 levels in the air inside a
building, and an air-handling system that uses data from the sensors to regulate the
amount of ventilation air admitted. 1 CO2 production is closely associated with the number
of people occupying a given space who exhale CO 2 as they breathe. Indoor
concentrations of CO2 are generally higher than the more diluted outside air, thus
measurement of indoor CO2 levels can be used to control the amount of outdoor air
entering a building to thereby adjust and dilute indoor CO 2 levels. Conventional DCV
systems introduce outdoor air into a building at a fixed rate, but CO 2 based DCV systems
can control ventilation rates at a cfm (cubic feet per minute) per person level that is based
upon real-time occupancy.
ANSI/ASHRAE Standard 62-2001 requires buildings to maintain a certain rate of
ventilation in order to maintain an acceptable indoor air quality level. Complying with
these standards puts operating costs in contention with maintaining proper ventilation
requirements. 2 Using CO2 sensors in conjunction with DCV systems manages spaces
where occupancy levels often vary, and are especially effective when occupancy levels
are lower than what the space was designed for. Over ventilation generally occurs in
these spaces, but CO2 sensors, which can be placed in ducts or on a wall-mount, can
eliminate an oversupply of outdoor air. This leads to energy savings because instead of
the ventilation system delivering a constant rate of outdoor air, CO 2 sensors adjust airflow
rates based on occupancy while still maintaining target cfm rates per person and
complying with Standard 62-2001.3 It costs more money to deliver a constant supply of
outdoor air into a building because the air needs to be conditioned, resulting in higher
energy consumption. Many buildings use DCV systems that deliver a constant rate of air
supply into a building even during low occupancy times, thus the air is being conditioned
in the building when no one is there. CO 2 based DCV systems will lower the amount of
outdoor air supply into a building, thus reducing costs associated with conditioning that
air.
1
U.S. DOE. "Demand-Controlled Ventilation Using CO2
Sensors."https://www1.eere.energy.gov/femp/pdfs/fta_co2.pdf (accessed August 10, 2010).
2
Penn State University. "Demand Controlled Ventilation." http://doas-radiant.psu.edu/IAQ7.pdf (accessed
August 10, 2010).
3
Is.mines.edu. "Application of CO2-Based Demand Controlled Ventilation Using ASHRAE Standard 62:
Optimizing Energy Use and Ventilation.
"http://www.is.mines.edu/plant/EEMS/energyconservationteam/Co2%20saving%20calculations1.pdf
(accessed August 10, 2010).
NJ GREEN BUILDING MANUAL
NEW COMMERCIAL UPDATED 5-MAY -11
How to Implement Demand Control Ventilation
Installing CO2 sensors for ventilation systems is starting to become more common with
over 60,000 units being sold annually in a continually growing market. For new
commercial buildings, installing a CO2 based DCV system will not add much difficulty
for the contractors during the commissioning process. Installing a CO 2 based DCV
system in a variable-air-volume (VAV) system in order to comply with ASHRAE 622001 is more complicated. This involves implementation of more complex algorithms in
order for the control system to work properly with the DCV. The benefits of this complex
strategy usually mean increased energy savings and improved indoor air quality. From a
maintenance standpoint, CO2 sensors are straightforward. Some manufacturers offer CO 2
sensors that can recalibrate themselves and are guaranteed to not need calibration for up
to 5 years. It is recommended though to check sensor readings during times when the
building is unoccupied and compare them with outdoor CO2 readings. If a sensor is
malfunctioning, most sensors will alert maintenance staff that there is an issue that needs
repairing. In addition, sensors are generally more effective as a wall-mounted unit but not
in a place where people can breathe directly onto them.
Examples
Summerfield Elementary School (Neptune Township, NJ)
http://www.neptune.k12.nj.us/11361023211333587/lib/11361023211333587/Summerfiel
d%20LEED%20Gold%20Flyer.pdf
30-Story Office Building (Birmingham, AL)
https://www1.eere.energy.gov/femp/pdfs/fta_co2.pdf (pg. 13-14)
Benefits
Employees generally spend around half of their waking hours at their workplace.
Conditioning the air that circulates throughout the building will help eliminate
contaminants and odors that can affect a person's work. Also, too much CO2 in the air
(from a lack of fresh air being introduced) not only makes the air seem stale, but it has
been linked with workers becoming lethargic and drowsy. This leads to decreased
productivity and quality of work produced. By having CO 2 sensors monitor the air
supply, re-circulated air can be adjusted to dilute CO 2 concentrations if they are becoming
NJ GREEN BUILDING MANUAL
NEW COMMERCIAL UPDATED 5-MAY -11
too elevated, thus maintaining better indoor air quality. 4 Additionally, if there is an over
provision of outdoor air in a humid climate, mold and mildew growth may occur which
negatively effects the IAQ of a building.
Costs
For new commercial building's HVAC systems, installed costs for CO 2 sensor technology
will generally range from $600 to $700 per sensing zone. An uninstalled CO 2 sensor
usually costs about $250 to $260 per sensor. Installation costs for a wireless system are
generally small compared to the actual cost of a CO 2 sensor, and a wireless system
connects multiple sensors. From case studies of CO 2 based DCV systems in large office
buildings, payback periods range from 0.4 to 2.2 years. Additionally, studies have shown
that there are greater savings potential for heating rather than cooling in all climate
types.5
As previously mentioned, CO2 based DCV systems have the potential to achieve the most
cost savings when implemented in buildings where occupancy rates vary throughout the
day and generally do not stay at full capacity. The estimated cost savings of a CO 2 based
DCV system has ranged from $0.05 to over $1.00 per square foot per year over
conventional DCV systems. Usually, higher density buildings with varying occupancy
rates achieve the higher end cost savings. Also, buildings with high cooling and heating
demands, as well as high utility rates, have more potential for better paybacks.
Resources
US DOE – Demand-controlled Ventilation Using CO2 Sensors
www.1.eere.energy.gov/femp/pdfs/fta_co2.pdf
Demand Control Ventilation White Paper
http://www.csu.org/business/greenback/energy/library/item2324.pdf
AIRCUITY
http://aircuity.org/Marketing/index.asp?p=Demand_Controlled_Ventilation
4
Washington State University Extension Energy Program "Why Measure Carbon Dioxide in Buildings?"
http://www.energy.wsu.edu/documents/building/iaq/CO2inbuildings.pdf (accessed August 10, 2010).
5
US DOE. Demand-controlled Ventilation Using CO2 Sensors.
www1.eere.energy.gov/femp/pdfs/fta_co2.pdf (accessed August 10, 2010).