“no less than one sensor per 10,000 ft² (930 m²) of floor space” and specifies that “CO2 sensors shall be located in the room between 3 ft and 6 ft (0.9 m and 1.8 m) above the floor or at the anticipated height of the occupants’ heads.”

Very few empirical data are available for guidance on proper selection of CO2 sensor installation locations. Multi-location and multi-height measurements of CO2 concentrations in meeting rooms [11] indicated that concentrations of CO2 at different wall-mounted sample points sometimes varied with location by more than 200 ppm. This research indicated that measurements of CO2 at return air grilles may be preferred to measurements with sensors located at walls because of the large possible range in concentrations that vary spatially, but measurements at return air grilles is not permitted in California’s Title 24. The research found that the exhaled breath of occupants in a meeting room can cause large fluctuations in the output of nearby CO2 sensors, thus, it is recommended that sensors be located away from locations where occupants are likely to sit or stand. The study found no consistent increase or decrease in CO2 concentrations with sensor height.

Figure shows example of the spatial variability of CO2 concentrations in a meeting room. "Wall" refers to locations on meeting room walls. "Return" refers to locations inside return air duct. 

See [11] for more information.