The Model 556 is a counter balanced rear flanged vertical mount automatic exhaust shutter. It is designed to allow horizontal airflow in one direction and to prevent reverse airflow while providing protection from environmental elements when the airflow stops. The blades open with airflow and minimize the wear load on the fan motor and close by gravity when the fan shuts down. counter balanced blades allow the shutter to be mounted vertically to 45 degrees and has flanged ends for added weather protection. Ideal for direct-drive or belt-driven exhaust fans. Do not use with vane axial or tube axial fans.