The most efficient chimney is one built perfectly straight with a round
or nearly round flue and a smooth interior surface. There is no advantage
in reducing the sectional area toward the top. The cross section and
height are determining factors. The transverse area must be sufficient to
pass the volume of air required to burn the fuel properly, and the height
must be great enough to insure against interference with the draft by
adjoining buildings or projections of the same building and to produce a
sufficiently strong draft.
Loss in draft strength is due to air leakage, and friction of the gases
against the sides of the chimney. A round flue (see fig. 1) is the most
desirable because it offers less resistance to the spirally ascending
column of smoke and gases. The elliptical is second choice so far as the
movement of the gases is concerned, but the difficulties that it presents
in manufacture and construction eliminate this shape. A rectangular
chimney either square or oblong is not effective over its full transverse
area; for the rising column, being approximately circular in section,
does not fill the corners. However, square or oblong forms are far
more common than the round, owing to the greater cost of round flue
construction. Square flues are preferable to oblong so far as efficiency
is concerned, but in the larger sizes of house flues the oblong shape is
more generally used because it fits to better advantage into the plan
of the house. An oblong flue should never have the long side more than
4 inches greater than the short side. A flue 8 inches by 16 inches is
bad flue construction for draft purposes. The sizes given in Table 1 are
recommended by the National Warm Air Heating and Ventilating Association.
Like all data for both high and low pressure flues, these sizes are based
on experience, not on scientific data, and are subject to modification by
further research. The dimensions given are for unlined flues. The actual
inside dimensions of flue tile are slightly different because of the lack
of standardization. In selecting the flue for a furnace or other large
heating unit an 8-inch by 12-inch size should be considered the minimum
for a lined or unlined flue, and 12 inches by 12 inches the minimum for
a lined or unlined flue whose height is more than 35 feet measured above
the grate level. If the chimney is designed for a small unit such as a
laundry stove or kitchen range an 8-inch by 8-inch flue may be used.
[Illustration: Fig. 2.--Top of chimney should be at least 2 feet above
the top of ridge in order that the wind currents may not be deflected
down the chimney.]
The proper size of flue depends upon the size of the heater or furnace
for which is to be used. All manufacturers' catalogues contain the size
of the smoke pipe for each particular heater, and from Table 1 (minimum)
dimensions for round, square, and oblong flues may be selected; or if the
catalogue contains stack sizes select the proper one. The flue tile to be
used should have a transverse net inside area approximately equal to that
of the smoke pipe.
Table 1.
Diameter Diameter
of smoke of smoke
pipe or Height of pipe or Height of
round Size of chimney round Size of chimney
chimney chimney flue above chimney chimney flue above
flue. flue. grate. flue. flue. grate.
----------+--------+-----------+---------+---------+----------
Inches. Inches. Feet. Inches. Inches. Feet.
8 8 by 12 35 15 16 by 16 45
9 8 by 12 35 16 16 by 18 45
10 12 by 12 35 17 16 by 20 50
11 12 by 12 40 18 16 by 20 55
12 12 by 12 40 19 20 by 20 55
13 12 by 16 40 20 20 by 24 60
14 12 by 16 45