Buildings Get a Breath of Fresh Air with CO2 Sensors

When it comes to energy use in buildings, emphasis is often placed on saving electricity by turning up the thermostat when running the A.C., or setting the thermostat a little lower when the heat is on. With the heating, ventilation, and air conditioning (HVAC) systems using the largest share of building energy, these are logical actions. But they only address the heating and cooling elements of the HVAC equation.

Carbon dioxide sensors help to fill the gap. At first glance, these small carbon dioxide sensors, which are no bigger than the palm of your hand, may seem an unlikely university ally in the fight to slow climate change, but CO2 sensors have long been known by facilities operators and engineers to save on HVAC costs. Since the mid-1990s, they have been facilitating significant energy savings at Xavier University and the University of Michigan.

Simply put, CO2 sensors measure the amount of carbon dioxide in a room. Too much CO2 and not enough oxygen indoors can cause a lack of concentration, sleepiness, and lethargy in building occupants. In order to prevent a buildup of carbon dioxide, building codes typically have specific ventilation requirements based on maximum occupation estimates per room.

“You’d be surprised how quickly a room full of people can drive up the carbon dioxide level in a room,” says Ray Hicks, president of

So, to ensure the student population can concentrate in class or cheer on their sports team, these sensors detect “the relative number of people in the building, allowing the building’s energy management system to trim the amount of outdoor air brought into the occupied spaces,” says Mark Hanlon, associate director for operations of the physical plant at Xavier.

“Most air conditioning and heating systems ventilate on a timed schedule, bringing in a set level of air based on the building code and regardless of the actual CO2 level in a room,” says Hicks. By using carbon dioxide sensors to facilitate on-demand ventilation, Hicks suggests that energy costs can be reduced by upwards of 30 percent.

At Xavier University, Hanlon says that when analyzing the energy cost savings brought by CO2 sensors, “You really do not want to separate ventilation savings from heating and cooling savings because a significant portion of the savings comes from not having to heat or cool the outdoor air you are ventilating with. For example, the HVAC system in our Gallagher Student Center, an 80,000 sq. ft built in 2002, uses approximately $72,000 a year in energy. Without the CO2 sensors the building’s HVAC system would consume an additional $16,000 in energy annually because it would operate as if the building was full of people.”

Encouraged by success in the student center, Xavier has expanded use of the sensors. “At first, we implemented CO2 sensors in the bigger gathering spaces like chapels, gyms, and large lecture halls. As [the sensors] have become more affordable and reliable in the last 10 years, we started installing them in individual classrooms,” says Sustainability Initiative co-chair Dave Lococo

A breakdown of energy consumption by building is detailed on the University of Michigan’s “Energy Management” website, including annual savings on utility bills. For example, in Chemistry Building #158, carbon dioxide sensors installed in the main lecture hall have resulted in an annual savings of $1,800.

Hicks points out the high savings of these systems: “CO2 monitoring systems are becoming increasingly popular. Now you can set up a small-scale system for less than $200.”

So, while carbon dioxide sensors monitor and adjust a building’s air intake based off of the CO2 emissions students breathe out, the sensors are also helping to tackle the bigger CO2 problem the world faces.

Plus, they are catching on in more places: “It’s not just universities that can benefit from using the sensors,” says Lococo. “As monitoring indoor CO2 becomes more ubiquitous and building code regulators increasingly adjust ventilation requirements based off of CO2 monitoring systems, more builders will realize the energy savings that accompany the use of carbon dioxide sensors.”

Haley Paul works at the University of Arizona Cooperative Extension, whose mission is to improve lives, communities, and the economy. She specializes in water resources and urban agriculture. Haley holds an M.S. in Sustainability from Arizona State University.

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Published: October 13, 2009