Brick chimney construction for furnace


Old 12-28-00, 07:43 PM
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In new home construction, is it typical to leave a gap between the back of a brick chimney and the wall sheathing? Primary interest is for a furnace chimney only.

The BOCA Mechanical Code seems to require a gap unless the chimney meets ULL specs. If the chimney is kept away from the sheathing, how is it typically connected to the house? Also, how is either vinyl or aluminum siding typically installed against the chimney to eliminate gaps?
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Old 12-31-00, 11:19 PM
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Depictions of modern exterior masonry chimney clearances (2" gap) and (1"gap)
Edge contact of exterior siding at a masonry chimney has been accepted by inspectors. Flashing, and/or trim, and caulking are the norm. An alternative is a chase, metal vent, with brick veneer.

An overview & summary of general requirements
For exterior "house connection" note reinforcing and anchorage paragraphs.

More information:
Brick Institute of America
BIA technotes (free online) provide an overview of empirical and applied principles.
Technotes: 19 (residential fireplaces), 19B (chimneys), 44 - 44B (anchor bolts - fasteners - wall ties).

National Fire Protection Association
Document - NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning, Appliances, 2000 Edition. (Purchase online [PDF format for immediate download]).

NFPA 211 is a comprehensive consensus guide of various regulations (56 pages, including header and index). The NFPA recommends use in conjunction with local codes. (When combine with local codes it would be difficult to err).

Masonry anchors
Old 01-03-01, 08:48 AM
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Brick chimney; gap to exterior wall

Thanks for the references. The BIA Technotes are very informative although it is interesting that none of the cross sections shown for chimneys show the relation of the brick to an exterior framed wall.

Based on discussions with several masonry contractors as well as code officials, there are a variety of opinions about proper design and construction, notwithstanding the building code requirements which clearly call for a gap between an exterior chimney and any combustible material on an exterior wall such as wall sheathing. All of the contractors have told me they build chimneys up against the wall sheathing.

In two cases I have investigated recently, it certainly apppears that the chimneys were built with a clear gap between the back of the chimney and the wall sheathing. Each house was about 30 to 40 years old. In each case replacement vinyl siding had been installed, with wide open gaps between the siding and the chimney. The gaps were full height and uniform, essentially ruling out settlement as the cause.

It would appear that the building code requirement for a gap is overly conservative considering that the brick around a flue liner would not get hot enough to cause any problems. Perhaps this is why building officials allow construction without a gap.

Old 01-03-01, 09:49 PM
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The controversy over proper chimney materials, construction, and clearance implementation seems to be never ending. The BIAs neutrality...choosing to parrot clearance standards is evidence of the saga.

I believe that the cited references are among the better overall guides. NFPA 211 is a comprehensive consensus but it deliberately side steps certain issues by suggesting use in conjunction with local codes (inferring local practice in the remotest sense only).

I think code conservatism reflects potential concerns (end user misuse and neglect); it lump sums various methods under a single general heading, and at times violations are tolerated. Legally, the definitive answer is strict code enforcement. But letter and sprit implementations will continue, and the saga continues. Your conversations are a reflections.
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