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Fireplace Chimney Proper Design Is Essential To Your Best Results

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

MATERIALS

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.

FOUNDATION

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.

FLUE

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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.

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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.

WALL STRUCTURE

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.

CAPPING

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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