The conventional Primary Air-handling Unit (PAU) is part of a central air-conditioning system used to supply fresh air to the floors it serves at a constant rate, regardless of the actual needs of the zones served.

Demand control on PAUs using carbon dioxide concentration level as an indicator reduces energy use while optimising indoor air quality. 

CO₂ based demand control is best applied to spaces with variable or intermittent occupancy, such as lecture halls, classrooms, conference rooms, theatres, waiting areas, and office spaces. By using Variable speed drives (VSDs) and CO₂ sensors to vary the air flow according to the actual demand, a significant amount of energy can be saved. In spaces without variable occupancy, CO ₂ control can also ensure that the space is ventilated at the appropriate level for its occupancy, rather than being ventilated at an arbitrary rate that was determined when the building was designed.