The sectional area of the flue bears a direct relation to the area of the
fireplace opening. The area of lined flues should be a tenth or more of
that of the fireplace opening. If the flues are unlined the proportion
should be increased slightly because of greater friction. Thirteen square
inches of area for the chimney flue to every square foot of fireplace
opening is a good rule to follow. For the fireplace shown in figure 13-A,
the opening of which has an area of 8.25 square feet, there is required
a flue having an area of 107 square inches. If this flue were built of
brick and unlined it would probably be made 8 inches by 16 inches, or 128
square inches, because brickwork can be laid to better advantage when the
dimensions of the flue are multiples of 4 inches. If the flue is lined
the lining should have an inside area approximating 107 square inches. It
is seldom possible to secure lining having the exact required area, but
the clear area should never be less than that prescribed above.
[Illustration: Fig. 13.--A. Top of throat damper is at DD, smoke shelf
at CO. Side wall should not be drawn in until the height DD is passed.
This assures full area. If the drawing in is done as indicated by lines
EF and EG, the width of the throat becomes less than the width of the
opening and causes the air currents to pile up in the corners of the
throat, resulting frequently in a smoky fireplace. B. Correct fireplace
construction.]
Failure to provide a chimney flue of sufficient sectional area is in many
instances the cause of an unsatisfactory fireplace. The cross section
should be the same throughout the entire length of the chimney. Do not
contract the flue at the chimney top, for that would nullify the larger
opening below; if it is necessary to change the direction of a flue the
full area should be preserved through all turns and bends, and the change
should be made as gradual as possible.