The best location for the chimney is near the center of the building, for
when so located its four walls are kept warm; cold winds can not chill
it and cause it to draw poorly. However, it is not always possible to
plan the arrangement of rooms so that the chimney may be thus located.
The outside wall of a chimney should be at least 8 inches thick in order
to reduce heat loss and the chance of air leakage into the flue.
[Illustration: 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.