Chimney Cross-Sections

The chimney cross-sections shown above are fabricated from sheet metal. Factory-built chimneys should not be confused with "metal chimneys". Metal chimneys, sometimes called "smokestacks," are single-wall, heavy gage (16 gage minimum) steel pipes. They are generally used in commercial application, and they are not allowed for use inside or outside one or two family dwellings.

There are two basic designs of metal factory-built chimneys from major suppliers; double wall and triple wall. A double wall chimney has an inner and outer wall; the space between is either filled with insulating material that has the ability to withstand very high temperatures, or is a ventilating channel.

Similarly, there are variations of triple wall chimneys. All have an inner and outer wall with a third wall separating the two. Triple wall types include air-insulated and air-cooled designs. Air insulated systems are designed to minimize air circulation between the walls, and between chimney sections. Air-cooled systems dissipate heat because air is thermo-siphoned through the channels between the walls. In order to fabricate a triple wall to meet the higher temperature requirements of UL 103 "Type HT", some triple walls now have a blanket of ceramic wool wrapped around the inner flue portion.

Factory-built types thus achieve close clearances to combustibles by means of an insulating layer of air and/or solid insulation between the hot interior flue wall and the exterior wall of the pipe.