To prevent exhaust air mixing with supply air by carryover, most installations have a purge sector. Its function is to flush the rotor matrix with outside air before it rotates into the supply air duct. In this way only outside air is present in the matrix and no carryover of exhaust air to supply air is possible. The purge sector is located on the supply discharge side of the supply air duct. Ostberg OCS casing is equipped with an adjustable purge sector for easy adjustment of the site request.