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. Sometimes it will work well and sometimes
it will not. The only safe rule is to have a separate flue to each
fire.

Another cause of poor draughts is too tight a room, so that the cold
air from without can not enter to press the warm air up the chimney.
The remedy is to admit a small current of air from without.

Another cause is two chimneys in one room, or in rooms opening together,
in which the draught in one is much stronger than in the other. In
this case, the stronger draught will draw away from the weaker. The
remedy is, for each room to have a proper supply of outside air; or,
in a single room, to stop one of the chimneys.

Another cause is the too close vicinity of a hill or buildings higher
than the top of the chimney, and the remedy for this is to raise the
chimney.