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Monitoring CO2 for Operational Cost Control

Until the recent growth of the green building industry under the LEED® program, the cost and time involved with using carbon dioxide sensors in commercial buildings often outweighed the perceived benefits. “Today, building owners and designers see more reasons than ever to include CO2 sensors in building design and remodels, as the cost and reliability have improved with the maturation of the technology over the last two decades,” says Jeff Hartnett, commercial split systems product manager for Lennox Industries.

While a certain level of CO2 is natural in buildings – humans produce it with every breath exhaled – high levels can cause a building’s occupants to become sluggish, tired and less productive. High CO2 levels may also indicate that a ventilation system is not operating properly and is allowing the buildup of high levels of pollutants such as volatile organic compounds.

CO2 sensing can provide a direct link to reducing operational costs.

When CO2 sensors are used with Demand Control Ventilation (DCV) systems, the amount of fresh-air ventilation is controlled based on how many people are actually in a space at any given time. When CO2 levels rise with increased occupancy, more fresh air is introduced by opening dampers. CO2 levels fall as people leave, so the dampers close to reduce the amount of air that must be heated or cooled and the costs associated with treating excess air. (Carbon dioxide is usually measured in parts per million, with outdoor levels typically around 400 to 450 ppm*, but ASHRAE Standard 62 should be consulted to calculate appropriate CO2 levels.)

Several considerations can help achieve the best results with CO2 sensing and Demand Control Ventilation:

  • The energy savings possible with CO2-controlled ventilation are highest in buildings that are irregularly occupied over a 24-hour period. That includes government facilities, theaters, nightclubs, restaurants, shopping malls and retail stores, classrooms or office buildings. Regions with high humidity are also appropriate for demand control ventilation, as limiting outdoor air can prevent an unnecessary influx of humidity that encourages mold and mildew growth.
  • To maximize control of operational costs, CO2 sensors can be tied into a building automation system or sophisticated zoning network such as the L Connection® Network via an intelligent direct digital controller. The controller allows the facility manager to monitor all devices from a single interface, providing information on CO2 levels, temperature, humidity, blower operation and other variables.
  • CO2 sensors must be located where they can sense true room conditions. If CO2 sensors are installed in ceiling air ducts, they can give inaccurate readings should air from the supply duct enter the return without circulating through the space. This means installing sensors in the area where people breathe: In an office building, for instance, that is three to six feet above the floor.
  • Choose a system that incorporates CO2, humidity and temperature sensing in a single device, such as the Comfort Sensor from Lennox. When a single zone sensor communicates all three levels, you can minimize installation labor and reduce wall clutter.
  • Sensors can reliably maintain their calibration, but only if they are calibrated correctly during installation. Inaccurate readings can affect occupants’ health and productivity and building energy usage.
  • Installation of CO2 sensors is not complicated. Other components, such as variable frequency drives and hardware to control input and output, may be needed and should be figured into the cost of the project.

Use of CO2 monitoring and Demand Control Ventilation can play an important part in an energy-efficient building design, and can help building professionals earn LEED credits for their projects.

*Federal Energy Management Program Federal Technology Alert: Demand-Controlled Ventilation Using CO2 Sensors, 2004.

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