TECHNOLOGY BASICS: What are Carbon Dioxide (CO2) Sensors?

Ventilation means providing outdoor air into indoor spaces and exhausting room air. Ventilation dilutes the concentration of indoor pollutants, such as volatile organic compounds and respiratory aerosols exhaled by building occupants. A standard ventilation system provides a fixed ventilation rate based on an occupancy schedule. Demand control ventilation (DCV) is an automated method of controlling outdoor air ventilation rate in response to actual building occupancy. A DCV system saves energy by reducing the ventilation rate, and thus, the amount of outside air that must be heated or cooled when the space is not fully occupied. DCV systems tested by the Product Evaluation Hub include two components: a carbon dioxide (CO2) sensor and a DCV controller.

Indoor CO2 concentration increases when occupants exhale CO2 and is reduced when outdoor ventilation air is introduced into the space. A minimum ventilation rate is required, even when CO2 levels are low, in order to remove building contaminants that are not a function of occupancy levels (e.g., off-gassing of building materials).

The indoor CO2 concentration in parts per million (ppm) is measured with a sensor and sent to a DCV controller. The DCV controller is programmed to maintain the indoor CO2 concentration at or below a setpoint. When the indoor CO2 concentration is below the setpoint, the ventilation decreases until the minimum ventilation setting is reached. When the indoor CO2 concentration is above the setpoint, the ventilation increases until the setpoint is met, or the maximum ventilation setting is reached. California’s 2019 Building Energy Efficiency Standards (Title 24) specify that CO2 levels must be maintained at 600 ppm above the outdoor concentration, which is assumed to be 400 ppm unless it is monitored (i.e. an indoor setpoint of 1,000 ppm).

In addition to being used in DCV systems, CO2 sensors are useful to monitor whether a space is sufficiently ventilated. When DCV systems are not in use, the ventilation system should be providing a fixed amount of outdoor air to the space based on the building’s design occupancy, which should result in CO2 concentrations that are below 1,100 ppm (assuming the average occupant produces CO2 at a rate of 0.01 ft3/min and that the fixed outdoor air rate is 15 CFM/person). CO2 sensors reporting concentrations above 1,100ppm indicate a problem with the building’s ventilation system, which increases risk of infectious disease spread.