The flexible ducts serve as a complement to ventilation ductwork. They are used for joining a series of rigid sheet metal ducts air valves or plenum boxes. They are also used for distribution of warm air from the ventilation cap and in heat recovery systems. The flexible duct is made of aluminium-polyester casing, reinforced with spirally wound steel wire. Due to their flexibility, the ducts can be deployed in hardly accessible places of the building and when it is difficult to plan how to connect boxes to the suspended ceiling.