Recirculation mode is used if your building does not allow air to be discharged into a chimney, i.e. typically in residential flats, prefabs and the like. In this mode, carbon filters are used to clean the air and remove odours. The cleaned air is then returned back into the room. Recirculation is slightly noisier than extraction.
In extraction mode, steam is extracted from the room to the outside of your home via a duct, chimney or ventilation shaft. For optimal performance and lower noise levels, the duct should be as short as possible and with a minimum of bends.
The backflow preventer is used when the hood in the air extraction mode. The preventer is installed on the vent termination and prevents the cold outside air from flowing into the room when the hood is venting the indoors air into the chimney.