Abstract: An air circulation system with standard and dual mode operations rapidly ventilates an enclosed area with fresh external air in the dual mode. In the dual mode of operation of the preferred embodiment, multiple barriers operate to create two airflow pathways. Two return air duct apertures are opened to expose the return air duct to an external air supply area via a fresh air duct. The fresh air duct includes an opening to the external air supply area and two extensions that are coupled to the return air duct apertures. The barriers simultaneously operate to block the return air duct between the two return air duct apertures to form two distinct return air duct segments. A central air unit and an exit fan, such as an attic fan, cooperate with the barriers to establish two airflow pathways through the return air duct segments.