A smoke shelf and chamber are absolutely essential. The shelf is formed
by setting the brickwork back at the top of the throat to the line of the
flue wall. The shelf should be the full length of the throat. The depth
of the shelf should be not less than 4 inches. It may vary from this to
12 or more, depending upon the depth of the fireplace.
The purpose of the smoke shelf is to change the direction of the down
draft so that the hot gases at the throat will strike it approximately
at a right angle instead of head on. Therefore the shelf should not be
bricked up as shown in figures 15 and 16, but should be made as wide as
the construction will permit at a height of 8 inches above the top of the
fireplace opening.
The smoke chamber is the space extending from the top of the throat up to
the bottom of the flue proper and between the side walls, which may be
drawn in after the top of the throat is passed. The area at the bottom
of the chamber is quite large, since its width includes that of the
throat added to the depth of the smoke shelf. This space is capable of
holding accumulated smoke temporarily in case a gust of wind across the
top of the chimney momentarily cuts off the draft. Smoke might be forced
into the room if there were no reservoir to hold it. The smoke chamber
also lessens the force of the down draft by increasing the area through
which it passes. If the walls are drawn inward 1 foot for each 18 inches
of rise, friction is reduced and interference with the draft lessened.
The walls should be smooth inside, for roughness seriously impedes the
upward movement of the air currents.