Match the diameter to the machine
Ducting has to fit the machine's outlet, or spigot, so start with its diameter: as a rough guide, around 200 millimetres for small heaters and fans, 250 to 300 for mid-size fans, and 400 to 600 for the largest ventilators and extractor fans. Some ducting is sized in inches, where 8 inches is about 200 millimetres and 12 about 300. Each machine also has a maximum ducting length, from 2 metres on a small heater to 20 metres on the portable extractor fans.
Fabric, aluminium or wire reinforced
Ribbed or wire-reinforced ducting holds its shape when a fan sucks air through it, so it works on the intake side of an extractor as well as the outlet. Insulated aluminium ducting keeps warm air warm along the run and cuts condensation, while flame-resistant coated fabric is the safer choice near welding and hot work. Fabric ducting also folds down small for storage between jobs.
Short runs and secure joints
Every extra metre and every bend cuts the airflow, so keep runs short and straight and pull the ducting out fully rather than leaving it bunched up. Secure each end over the spigot so it cannot work loose, and check for splits before each job, because a leak on an extraction run lets fumes back out. Dry fabric ducting before folding it away to stop mould and smells.
