Backdraught shutters for ventilation — preventing air backflow when the fan stops

A backdraught shutter is a passive device — no motor, no electrical supply — that opens in the normal airflow direction (when the fan operates) and closes automatically when reverse pressure occurs (when the fan stops or when external pressure exceeds installation pressure). Mandatory at any wall-mounted exhaust fan on an exterior wall, at installations with multiple fans on a shared collector, and at bathroom/sanitary exhausts connected to a shared riser duct. Available in circular (direct mounting on spiral duct, all standard diameters Ø100–500 mm) and rectangular forms, with plastic blades (light, residential, low pressures) or galvanised sheet metal blades (robust, industrial, higher pressures).