Terracotta Air Vent & Back Draught Shutter For 4 Inch (100mm) Extractor Fan Ducting Gravity Flap Vent Wall vent with gravity flaps and weather sill. Helps prevent backdrafts. Manufactured with UV stable materials to reduce colour fade. Suitable for round 100mm (4 Inch) ducting. Overall size: 140mm x 140mm. British made. ABS thermoplastic. Complies with Part F building regulations. Suitable for commercial and domestic use. Other vents and colours available in our shop. Back Draught Shutter In-line spring operated back draught shutter. Stops back draught and wind noise when a fan is switched off. Made from tough, white GRP plastic. Fits Inside standard size 100mm diameter ducting. Fit the shutter with the centre spindle of the flaps in a vertical position and make sure ducting is level. Other ducting products availabe in our shop. Please note that these shutters have 1 spring and not 2. This is correct as 2 springs would offer too much resistance. ABS thermoplastic. Complies with Part F building regulations. Suitable for commercial and domestic use.