The following are some of the causes and the remedies for this evil.
The most common cause of poor chimney draughts is too large an opening
for the fireplace, either too wide or too high in front, or having too
large a throat for the smoke. In a lower story, the fireplace should
not be larger than thirty inches wide, twenty-five inches high, and
fifteen deep. In the story above, it should be eighteen inches square
and fifteen inches deep.
Another cause is too short a flue, and the remedy is to lengthen it.
As a general rule, the longer the flue the stronger the draught. But
in calculating the length of a flue, reference must be had to
side-flues, if any open into it. Where this is the case, the length
of the main flue is to be considered as extending only from the bottom
to the point where the upper flue joins it, and where the lower will
receive air from the upper flue. If a smoky flue can not be increased
in length, either by closing an upper flue or lengthening the chimney,
the fireplace must be contracted so that all the air near the fire
will be heated and thus pressed upward.
If a flue has more than one opening, in some cases it is impossible
to secure a good draught.
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