The hood has a non-return valve (flap) that prevents cold air from entering from the outside. When it blows properly in the chimney, a negative pressure is created which causes the flap to be lifted.
The discussion revolves around the issue of a hood being turned off while experiencing knocking sounds during windy conditions. Possible causes include increased chimney draft causing movement within the hood or chimney. Solutions suggested include cleaning the chimney if the issue is located there, or identifying and repairing the specific component causing the noise within the hood. A non-return valve (flap) in the hood may be lifting due to negative pressure created by wind, which can lead to knocking sounds. For new hoods, it is recommended to check for the presence of such a flap. A practical solution offered is to cushion the impact of the flap using self-adhesive sponge material. Summary generated by the language model.
