Abstract: An air distribution system employs an inner proportioning pipe inside an outer proportioning pipe. Airflow into the proportioning pipes is adjusted by controlling the position of the outer proportioning pipe with respect to the walls of a duct. Proportioning of airflow between the inner and outer proportioning pipes is adjusted by controlling a transverse or an axial relationship of the inner and outer proportioning pipes. Air flowing in the inner proportioning pipe is added to and mixed with air flowing in the outer proportioning pipe in an air balancing chamber. The mixing is performed by rotating vanes. Mixed air from the air balancing chamber is passed through a plurality of discharge openings. The proportioning and mixing provides a predetermined relationship between the airflows at the discharge openings.