Heavier oil fumes will settle at the bends of the vertical and horizontal flue pipes, with a smaller portion remaining on the inner walls of the vertical flue without forming a solid. If the cleaning of the flue pipes has not exceeded six months, there is no need to clean the external vertical flue, as the lighter fumes have been vacuumed away by the fan. Therefore, for flue pipes that have not exceeded six months, only the bends of the flue need to be cleaned. It is recommended to clean the flue pipes every six months under normal circumstances.