A fireplace chimney is a necessity with
any fuel-burning equipment. It must be capable of producing and maintaining
a draft to bring a supply of fresh air to the fire. Also, it mast carry
away particles and harmful gases emitted from the burning fuel. The chimney,
and the flue within the chimney through which these gases travel, must be
carefully built to be free of fire hazards. A defective or overheated chimney
is the greatest single known cause of residential fires
The common brick is suitable for fireplace chimney construction, but firebrick
should be used for the inside course, if a flue lining is not used. Firebrick
resists heat and temperature changes more readily than ordinary brick. Stone
can also be used, but the necessary thickness of the wall often is not desirable.
Concrete block can also be used, and solid block units can be used to erect
chimneys of any size and shape. These units are sized so they combine easily
with rectangular and circular linings.
A solid concrete foundation, 36" deep and extending 6" wider and longer than the chimney plan, is usually sufficient. Since regional requirements differ, consult your local building code. To determine the weight of the chimney, figure brick at 130 pounds per cubic foot and concrete at 150 pounds per cubic foot. Figure entire cross section cubage.
The flue lining must be capable of withstanding rapid changes in tem perature. The liner used should be made of fire clay not less than five-eighths of an inch thick. Cracks or other imperfections in any part of the section make the whole length unfit for use. Brickwork and mortar joints have a tendency to disintegrate when exposed to flue gases. This, combined with natural weathering, will cause cracks in the masonry, reducing the draft and causing fire hazards. So whenever a flue lining is omitted, an eight-inch thickness of masonry wall should be used.
OPENINGS INTO CHIMNEY
Any openings in a fireplace chimney should consist of a metal thimble
around which the brickwork is carefully laid and cemented. Boiler clay or
putty should be used to make any loose smokepipe connections air-tight.
The walls of both exterior and interior chimneys without linings, and exposed walls of exterior chimneys, must be at least eight inches thick The walls of the interior chimney with flue linings must be at legist four inches thick. Interior chimneys should have eight-inch walls from below the roof to the top. Flue linings should be separated by brick divisions or withes at least four inches in width- The walls of stone chimneys should be at least 12 inches thick.
CHIMNEY AND ROOF
Careful consideration should be given to the proper connection between chimney and roof. This is one of the principal points at which a fire can start. Therefore, at least a two-inch clearance should be provided for between the wood framing and the masonry. This will also allow for expansion or movement of the chimney due to temperature changes or strong winds.
A chimney cap should always be provided so ;is to reduce fireplace chimney
maintenance. The top surface of the cap should slope upwards to the flue
to provide water drainage, and to deflect air currents upward as they pass
the chimney top. When a hood is used, the area of the hood opening should
be at least equal to to the area of the flue. Concrete and brick caps are
usually made four inches thick, and it is advisable to project them an inch
or two toform a drip ledge.
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