Central air conditioning is a system which delivers cooled air to multiple rooms at once through ductwork. It uses return air ducts to collect air from various rooms, and redistributes freshly cooled air through air supply ducts. It often enhances the air quality in a building through filters which remove dust and pollutants. It usually permits at least some local temperature adjustment. The system requires a condenser/compressor unit and an evaporation unit, sometimes combined into a package unit, which is normally placed outside, on the roof, or on the facade.

The cooled air is distributed through supply ducts to several rooms, and return ducts bring air back for cooling. This common air-distribution arrangement distinguishes the defined system from separate room units serving individual spaces.

Cooling ceilings circulate chilled water through ceiling pipes or coils. The central air-conditioning system described here circulates cooled air through ducts, while direct digital control concerns the controls operating HVAC equipment.