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Single wall metal chimneys are simply
unacceptable. Metal chimneys serving appliances with flue gas
temperatures at the entrance to the chimney below 350 degrees should be
lined with an acid and condensate resistant metal or refractory material.
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Clearances
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Metal chimneys should have enough
clearance to avoid heating combustible materials to a temperature in
excess of 90 degrees above the ambient temperature.
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Exterior and Interior. Wood
frame or other combustible material—18 inches minimum.
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Exterior and Interior. Metal
chimney over 18˛ diameter—Non combustible materials—4 inches
minimum.
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Exterior and Interior. Metal
chimney 18˛ diameter or less—Non combustible materials—2 inches
minimum.
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Interior. Metal chimneys must
be enclosed in a continuous enclosure and maintain the integrity of
fire separations—1 hour rating in buildings up to 4 stories.
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Fire stops at floors, walls and
roofs must be made of entirely non-combustible materials. 9 inches is
the typical clearance, however, it is 18 inches if galvanized steel or
approved corrosion-resistant metal is not used.
Specific
Metal Chimney Concerns
Metal chimneys will dissipate heat much
quicker than masonry chimneys, even insulated metal chimneys, however, to
a lesser degree. This causes two issues, and both are related to the
draft.
1. The draft
in metal chimneys is more difficult to get started. The colder air in
the chimney reduces the ability of the smoke to rise, because it is
heavier than the smoke. Once the fire is going, the chimney is warmed;
there is no problem until the fire starts to go out and the air in the
chimney cools.
2. When
the fire goes out, and only smoking embers remain, the cooler air starts
to accumulate in the top of the chimney. If the outside
temperature is low enough, the cold air may cause the last 30 to 45
minutes of smoke to back into the house.
This condition is worse when the
outside temperature is below 20 degrees Fahrenheit, and when the chimney
is on the outside of the house. When the outside temperature is above 30
degrees Fahrenheit, this is generally not a concern. There is also less
concern when the metal chimney goes through the interior of the house.
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Chimney
Main Height
of Chimneys Brick
Chimneys Masonry
Chimneys
Backdrafting Vent
Connectors Common
Defects
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