Abstract: An air circulation system is provided for a building having a plurality of zones to be heated, ventilated or cooled. The system includes an air circulating apparatus arranged to supply either heated, cooled or ventilated supply air to the various zones of the building via a plurality of supply ducts. The apparatus also draws return air from the zones via a plurality of return ducts. A damper is coupled to both the supply and return ducts of each zone, adjacent the air circulating apparatus, to balance the amount of supply and return air in each zone for reducing mixing of air between zones and stratification within a zone. Accurate and independent control of each zone in relation to the other zones in the building is thus permitted. The circular ducts are formed of sealed PVC or other suitable plastic material to improve the efficiency of the air delivery system both by reducing pressure losses and temperature losses conducted through the walls of the ducts.