Fig. 5.—A. An unlined chimney before use. B. Same chimney, after being in service. Frequently the heat and weather cause the mortar to disintegrate so that air leaks in through the joints, causing a reduction in the draft. C. Same chimney as A, showing terra cotta flue lining in place. D. An unlined chimney with offset. Loose brick and mortar may fall and become lodged at the offset during construction or loosening of the points and disintegration may cause bricks from an uncapped chimney to check the draft completely.
If the flue is lined and the chimney is not higher than 30 feet, its walls, if of brick, may be made 4 inches thick, provided adjacent inflammable material is properly insulated. If unlined, the walls should not be less than 8 inches thick. It is not good practice to place the linings of two flues side by side. If there is more than one flue in a chimney, the flues should be separated from each other by a division wall of brick at least 4 inches thick (see [fig. 6]), bonded into the side walls, and the joints of the flue linings should be staggered or offset at least 6 inches (see [fig. 7]). This construction insures stability, reduces the chance for air leakage between flues, and prevents the possibility of a fire in one flue involving an adjacent flue. If stone is used in chimney construction, the walls should be at least 4 inches thicker than brick walls.
Walls of concrete chimneys should be not less than 4 inches thick or else they should be reinforced in both directions; otherwise cracking during the setting of the concrete or, later, due to temperature changes or unequal settlement of the foundation is apt to occur. Concrete blocks are not recommended, but if they are used each block should be reinforced with steel running continuously around it and the blocks should be not less than 4 inches thick. They should be lined with the best flue lining. All monolithic concrete chimneys with walls less than 8 inches thick should be lined.
It is not unusual to find an opening into a chimney other than for the smoke pipe of the main heating apparatus. This is a frequent cause of unsatisfactory operation. No range, stove, fireplace, or ventilating register should be connected with the chimney flue built for the heating apparatus. If it should be desired to use an existing abandoned fireplace chimney for a range or stove the fireplace flue should be closed tight about a foot below the place where the smoke pipe enters.
