Carbon dioxide (CO2) sensors can be installed in areas that have infrequent or irregular occupancy patterns, such as auditoriums and classrooms. The sensors allow the ventilation system to be controlled by the concentration of CO2 present in the space, thereby reducing the amount of ventilation required during unoccupied times. As the space becomes occupied, the sensors will detect an increase of CO2 and adjust the amount of outside air needed. CO2 sensors, as well as other energy saving equipment, were installed in the Power Center for the Performing Arts on the University of Michigan's Ann Arbor Campus in 1994.


Applicable Regulations


Overview of Procedure
Two CO2 sensors were installed in the Power Center for the Performing Arts, one in the return air ductwork and one in the auditorium. As the auditorium space becomes occupied, the sensors detect the increase of exhaled CO2 from the occupants and direct the heating, ventilation, and air conditioning (HVAC) system to increase the amount of outside air used. Conversely, as the occupancy decreases and the CO2 levels decrease, the amount of outside air used will also be decreased. This significantly reduces the amount of energy needed to heat or cool the space.


Waste Minimization Procedure
The reduction of energy consumption directly correlates with reduced emissions of atmospheric pollutants that are produced as by products during energy production at power plants.


Known Limitations
CO2 sensors may not function with all types of HVAC systems.

The system is designed to work efficiently in large open spaces including auditoriums, classrooms, or conference rooms. Placement of sensor locations within a given space is critical for the system to work properly. If the sensors are not located correctly, fluctuations in temperature and overall comfort within the room may be observed.

In addition, errors in measurement and varying CO2 concentrations over time can cause low readings that may be misleading. Elevated CO2 may be due to various causes alone or in combination, such as: increased occupant population, poor air distribution, and poor air mixing. Smoke tubes and temperature profiles may help to clarify air circulation patterns.


Safety & Health Precautions/Personal Protective Equipment
Follow all applicable safety and health protocols and regulations as established by your institution.


The benefit of using CO2 sensors in infrequently or irregularly occupied spaces eliminates the need to continually supply the space with heated or air conditioned air. The implementation of CO2 sensors may significantly reduce costs associated with energy consumption needed for heating or cooling.


The sensors are not designed or intended for use in safety systems where personal injury may result.

Periodic maintenance of the sensors is required to ensure that acceptable air quality is maintained.


Project Related Costs
The costs associated with each sensor installation in 1994 was approximately $700 for the equipment and approximately $1,000 for the engineering and labor costs. It is expected that a payback can be realized in three to six months. The actual payback cannot easily be calculated for the Power Center for the Performing Arts as other energy saving devices, including Direct Digital Controls and new cooling tower and chiller interconnects, were installed in conjunction with the CO2 sensors. The CO2 sensors can be operated as a single energy savings device, although other upgrades were warranted in this building.

The overall calculated energy savings, not directly related to the CO2 sensors, for the Power Center for the Performing Arts in fiscal years 1995 and 1996 were $29,000 and $32,000, respectively.