In figure 13-B is shown the throat, the narrow opening between the
fireplace and the smoke chamber. Correct throat construction contributes
more to efficiency than any other feature except proper flue design.
A flue twice as large as is necessary brought straight down to the
fireplace without constriction at the throat would result in a poor
draft, for the draft does not depend upon the largeness of the flue but
upon its proper proportioning to the fireplace and throat. The arrows
indicate the upward flowing currents of warm air which are thrown forward
at the throat and pass through the smoke chamber into the flue on the
inner side. This rapid upward passage of air causes a down current on the
opposite side, as indicated by the descending arrows. The down current
is not nearly as strong as the up current, but it may be of such force
that if there be no throat to the fireplace (see fig. 14) to increase the
velocity of the upward current by constricting it, the meeting of the
two currents will result in smoke being forced out into the room. Thus
it frequently happens that a fireplace has an ample flue area and yet
smokes badly. The influence of the throat upon the upward and downward
air currents is shown in figure 13-B.
[Illustration: Fig. 14.--Fireplaces constructed like this without throat
will very likely smoke.]
The area of the throat should not be less than that of the flue. Its
length should always be equal to the width of the fireplace opening.
(See fig. 13-A.) The sides of the fireplace should be vertical until the
throat is passed. (DD in fig. 13-A.) Above the throat the sides should
be drawn in until the desired flue area is attained. The throat should
be set 8 inches above the location of the lintel, as shown in figure 13,
A and B. The wrong way to place the throat damper is shown in figure
15. The throat should not be more than 4 or 5 inches wide. The lesser
width is a safe standard. If a damper is installed the width of the
brick opening at the throat will depend upon the width of the frame of
the damper, the width of the throat proper being regulated by the hinged
cover of the damper. If the throat damper is omitted the opening should
be 4 inches, as shown in figure 16. The smoke shelf should not be bricked
up but should conform to the dotted lines. The depth of the smoke shelf
should be the same for a 2-foot as for a 10-foot fireplace opening.
Proper throat construction is so necessary to a successful fireplace that
the work should be carefully watched to see that the width is not made
more than 4 inches and that the side walls are carried up perpendicularly
until the throat is passed, so that the full length of opening is
provided. All masons do not appreciate these fine but necessary points.
Many prefer their own and sometimes will ignore the proper methods. It
is therefore advisable to inspect the work several times a day as it
progresses and thus avoid poor results. When trouble is experienced in an
existing fireplace that has ample flue area, it is usually found that the
formation of the throat is the cause.
[Illustration: Fig. 15.--Wrong location for throat damper. The throat is
so low that the accumulation of gases at the point constricted weakens
rather than improves the draft with greater likelihood of a smoky
fireplace. Note that the smoke shelf is bricked up. This is wrong.]