In Table 1 the minimum height of the chimney above the grate is given
as 35 feet. Higher chimneys are considered more satisfactory, and
authorities claim that any flue under 40 feet in height will produce
an erratic draft, good on some days but poor on others The force or
direction of the wind may be the cause, or the amount of moisture in
the air, or the quality of the fuel may be responsible. The higher the
chimney the less will be the possibility of counter air currents and
the stronger and more constant the draft. Soft coal and the sizes of
hard coal known as pea and buckwheat are apt to cake and fill up the air
spaces through the bed of the fire, with the result that an intense draft
is required to give the fuel sufficient air.
The top of the chimney should extend at least 3 feet above flat roofs and
2 feet above the ridge of peak roofs (see figs. 2 and 3), and it should
not be on the side of the house adjacent to a large tree or a structure
higher than itself (see fig. 4), for these may Cause eddies and force air
down the chimney. A poor draft will most likely result when the wind is
blowing in the direction indicated.
[Illustration: Fig. 3.--Extensions to the chimney required In order that
it might draw properly.]