A 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 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.
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FLUE
The flue lining must be capable of withstanding rapid changes in temperature. 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.