Although chimneys are built unlined to save expense, those properly
lined with tile are undoubtedly more efficient. Linings prevent
disintegration of mortar and bricks through the action of flue gases.
This disintegration and that occurring from changes in temperature result
frequently in open cracks in the flue (see fig. 5-B) which reduce or
check the draft. If loose brick and mortar should fall within they may
lodge so as to cause partial or almost complete stoppage (see fig. 5-D).
The danger of this latter condition is greater if the flue be built
with offsets or bends. Any change in direction should be made as gradual
as possible and with an angle not greater than 30 degrees with the
perpendicular.
The most important requirement for a flue lining is that it withstand
high temperatures and not be subject to disintegration by ordinary flue
gases. It should be made of fire clay and for the purpose. The thickness
should be 1 inch. It should be set in cement mortar with the joints
struck smooth on the inside. Each length of flue lining should be placed
in position, and the brick should then be laid around it; if the lining
is slipped down after several courses of brick have been laid, the joints
can not properly be filled with mortar and leakage is almost sure to
result.
[Illustration: Fig. 4.--Large trees located near chimney tops may deflect
wind currents down the chimney. This may be avoided by placing the
chimney on the opposite side of the building.]
Well-burned clay flue linings are generally satisfactory for
dwelling-house chimneys used for stoves, ranges, fireplaces, and
furnaces. In regions where the fuel is natural gas, hot flue gases are
said to have caused linings to disintegrate and crumble off. In such a
case it may be necessary to use a fire clay that has stood the test or
line the chimney with fire brick.
Linings are manufactured in round, square, and oblong shapes, but not
in elliptical. The oblong and square shapes are better adapted to brick
construction than the round. They permit of simpler and less expensive
masonry work. On the other hand, the round shape produces better draft
and is easier to clean.
A fireplace flue, if straight, should be lined from the throat
continuously to the top. The smoke chamber should be lined with fire
clay or cement mortar one-half inch thick. In case the masonry in front
of the throat is less than 8 inches thick the lining should start at the
bottom of the lintel. The hottest part of the flue is at its throat, and
if it is not lined at that point or if the masonry is not of sufficient
thickness, there is danger of overheating. Careful attention should be
given to details of flue construction in order to assure satisfactory
operation and reduce the fire hazard.