Messrs. W. H. Jackson & Co., of this city, whose long experience in
treating refractory flues gives weight to their opinion, communicate
to the _American Architect_ the following useful information:
To secure a good draught the chimney should be of sufficient size,
should be carried up above surrounding objects, should be as straight
as possible throughout its length, and should be as smooth as possible
inside, to avoid friction. As a draught is caused by unequal
temperatures, the chimney should be so arranged as to avoid a rapid
radiation of heat. If in an exterior wall there should be at least 8
inches of brickwork between the flue and the exterior surface. For
country houses it is much better to have the chimneys run up through
the interior, as the flue is more easily kept warm, and the heat that
is radiated helps to warm the house. The most frequent cause of a
"smoky chimney" is the insufficient size of the flue for the grate or
fireplace connected therewith. The flue should not be less than one
eighth the capacity of the square of the width and height of the grate
or fireplace. That is, if the grate has a front opening 20 inches wide
and 26 inches high, the flue should be 8 in. × 8 in.; or, with an
opening 36 inches wide and 32 inches high, the flue should be 12 in. ×
12 in.; and, to get the best result, the opening into the flue from
the grate or fireplace should be of a less number of square inches
than the square of the flue, and never larger, as no more air should
be admitted at the inlet than can be carried through the flue. Where
there is more than one inlet to the same flue, the sum of all the
inlets should not more than equal the size of the flue. A number of
stoves may be connected with the same flue, one above another, if this
rule is observed.
A square flue is better than a narrow one, as in two flues containing
the same number of square inches the square flue would have the
smallest amount of wall surface, and consequently less friction for
the ascending currents, and less absorption of heat by the walls.
Chimneys should be closely built, having no cracks nor openings
through which external air may be drawn to weaken the draught. If they
could be made throughout their length as impervious to air as a tube
of glass, with interior surface as smooth, one cause of smoky chimneys
would be removed. A downward current of air is frequently caused by
some contiguous object higher than the chimney, against which the wind
strikes. This higher object may sometimes be quite a distance from the
chimney, and still affect it badly. A good chimney top constructed to
prevent a down draught will remedy this difficulty. Each grate or
fireplace should have a flue to itself. Under very favorable
conditions, two grates or fireplaces might be connected with the same
flue, but it is not a good plan. We have known grates and fireplaces
connected with two flues, where they have been built under a window
for instance, and, owing to there being insufficient room for a flue
of suitable size, a flue has been run up on each side of the window.
This is a very bad plan, and never can work well; it requires too much
heat to warm both flues, and if the room in which the grate or
fireplace is situated should be pretty close, so that there was no
other entrance for air, there is danger that it would circulate down
one flue and up the other, forcing smoke out of the fireplace into the
room.
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