The draft depends entirely upon the chimney flue. The better the flue
the more satisfactory and efficient will be the operation of the entire
heating apparatus. The strength or intensity of the draft is dependent
mainly upon the tightness, size, and height of the chimney flue. The most
common error in chimney construction is failure to distinguish between
the size of flue necessary for free passage of the volume of smoke from a
given amount of fuel and that which with proper height will produce the
required draft. A chimney may be high enough, yet have an area too small
to carry properly the volume of smoke. On the other hand, the size may be
sufficient but the chimney too low to produce a draft strong enough to
pull the air through the fire at a sufficiently rapid rate. Either fault
or a combination of the two will result in unsatisfactory service.
Draft in a chimney flue is caused by the difference in weight between
a volume of air on the outside and an equal volume of products of
combustion from the fire on the inside. The higher the temperature of
a given weight of air, the greater is its total volume and the lighter
the weight of its unit volume. This produces a condition of unbalanced
pressures at the base of the flue. The rising of the lighter gases within
the chimney tends to equalize the pressures. So long as the fire burns
this condition of unbalanced pressure persists, the result being draft.
This is the basic principle which governs chimney action and upon which
the draft depends. The greater the difference between the temperature in
the flue and that outside the greater the tendency toward equalization of
pressure and hence the better the draft. In summer the draft of a chimney
is not as good as in winter because the difference in temperature between
the outside air and that of the gases in the flue is less.
[Illustration: Round. Elliptical. Square. Oblong.
Fig. 1.--Round flues offer the least resistance to the passage of gases,
but most residence flues are made either square or oblong for structural
reasons.]