CO2 sensor location

Even if there is no dilution of the return air from other return ducts before the CO2 sensor's location in the duct, LEED does not allow installing CO2 sensors in the return duct. (There is a CIR under LEED NC 2.2 (6/27/2008) that confirms the same.) We cannot find any exceptions to this. If a space does not have well mixed air (which is possible for various reasons) then the CO2 concentration may not be uniformly distributed. In such situations, locating a CO2 sensor in a return duct may not depict real conditions in the breathing zone.

It is at the designer's discretion to determine the quantity and location of multiple Co2 sensors. It depends on configuration of the space , distribution of population etc. There is some guidance in Appendix A of ASHRAE 62.1 but it does not address the quantity of Co2 sensors required in such spaces.