The draft of a chimney is the current of air created by the difference
in pressure resulting from variation in weight between the relatively
hot gases in the flue and the cooler outside air. The strength
or intensity of the draft depends, for the most part, on the height of
the chimney, and the temperature difference between the chimney
gases and the outside atmosphere. The draft is not so good in summer
as in winter because the difference in temperature between the
outside air and the gases in the flue is less.
A very common error in chimney design is failure to distinguish
between the size of the flue required 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
(fig. 3), yet have an area too small to expel the volume of smoke; or
the size may be ample (fig. 4) but the height not great enough to
produce a strong draft. Either fault or a combination of the two will
result in unsatisfactory service.