The building sector is considered one of the leading sectors in energy-consumption. In addition to lighting, heating and cooling systems have become important consumers in all buildings (both residential, administrative, and service both private and public) ensuring a comfortable environment for staff and workers and improving productivity at the lowest possible cost. This research aims to study the control quantities of fresh and exhausted air in large halls and auditoriums, where the occupancy level changes. This is done by adjusting the amount of fresh and exhausted air according to the occupancy via CO2 sensors, which measure the ratio of CO2 in the targeted places, controlling the volume of fresh and exhausted air according to the number of available people. A case study was done to use control systems; final results and conclusions have been illustrated.