Abstract: Flue sections and an airtight venting system being made therefrom by connecting in line the flue sections. The flue section has a pair of opposite ends of circular cross-section one of which called "male end" is shaped and sized to fit into the other end, called "female end" of a similar flue section. The male end comprises at least two outwardly-projecting L-shaped grooves, a peripheral outwardly-opening recess and an O-ring mounted therein and protruding outwardly. The L-shaped grooves are positioned in such a manner to have an opening at the male end of the flue section. The recess is positioned at a distance "H" from the L-shaped groove.