Article and photos by Greg Havel
During the 1800s and in the early 1900s, masonry chimneys for kitchen ranges and space heaters were often built on wood brackets or shelves attached to wood stud walls. This method saved the cost and labor of installing the “extra” masonry from the foundation to a point halfway between floor and ceiling. When these chimneys were no longer needed, the stove-pipe openings were closed with decorative tin plates. Many of these chimneys are still present in older homes and other buildings, even though they have been concealed by remodeling and reroofing projects.
In modern houses of lightweight wood-frame construction, chimneys for fireplaces and wood-burners are again being supported by wood framing. Although the manufactured double- or triple-wall metal chimney is lighter than a masonry chimney, so are its supports. Photo 1 shows a section of double-wall metal chimney with a support flange to be used where the chimney passes through a floor-ceiling assembly or below a roof. These floor-ceiling and roof assemblies today are usually supported from lightweight manufactured wood products like I-joists and wood trusses.
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Note that the interior pipe for combustion products is supported from the outer pipe by thin strips of sheet metal and rivets, as shown in photo 1. This space between the inner and outer pipe is open at the bottom of the chimney and is vented separately from the combustion products by a cap above the roof (photo 2). Convection in this space keeps the outer pipe comparatively cool and unlikely to ignite surrounding wood framing if the chimney is installed with the clearances required by the chimney manufacturer and by National Fire Protection Association 211 Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances.
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These manufactured metal chimneys are usually installed inside wood-frame chimney enclosures that extend from the floor at the fireplace or wood-burner, up through the roof. They may also be installed through exterior walls and exposed to a point above the roof. Photo 3 shows steel-stud framing for a chimney enclosure for a fireplace with a support flange near the top of the photo. This enclosure extends above the roof and appears as a rectangular box finished with siding to match the rest of the house, with the vent cap on top. These enclosures may also be clad with other materials, including brick or stone masonry, supported by the roof trusses and wall framing.
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- if they are used for higher temperatures than those for which they are rated,
- if modifications are made during installation of the chimney,
- if the chimney sections are not connected properly or if pipe sections from more than one manufacturer are used,
- if proper clearances are not maintained between the chimney and combustible building materials,
- if they are not properly supported, and
- if they are not cleaned regularly.
Internet search for “class a chimney pipe,” “double wall chimney pipe,” and “triple wall chimney pipe” for manufacturer Web sites with typical installation diagrams that conform to NFPA and ANSI/UL standards.
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