I'm trying to get an idea of where CO2 sensors would be required on a project. It is an office building with one AHU per floor and one for a fitness center. The building contains several densely occupied spaces (conference rooms and the fitness center).

It is unlikely that any of the AHU's are predominantly serving densely occupied space (except for the fitness center AHU). Assuming that this is the case, I believe that the fitness center is the only space that would require a CO2 sensor. Is this correct?