This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Chimneys are closed channels built of some incombustible material, and are used to cause the draught of air necessary for the proper combustion of fuel in the fireplaces below, and for carrying away the noxious fumes of combustion and discharging them at such a height that they may not be a nuisance to people living in their vicinity.
It is usual, for the sake of economy, to so place the chimney openings on plan that those on the same floor are near one another, and those on different floors over another, so that the flues may be carried up in groups or stacks. Some of the different methods of arranging these openings will be seen from the various plans in Part I. of this Volume.
The size of chimney openings is regulated chiefly by the size of the grate it is proposed to use. Kitchens require a width of from 3 ft. to 4 ft. 6 in. and a depth of about 1 ft. to 10 1/2 in., while ordinary grates require a width of 2 ft. 5 in. and a depth of 9 in.; though small bedroom grates are obtainable for openings of even so little as 9 inches. The Breast or brickwork above the chimney opening is usually supported on an arch of brick or stone, or upon a bar of wrought iron, and if the breast project more than 4 1/2 inches from the face of the wall, and the jamb on either side be less than 18 inches, the abutments should be tied in by iron bars built into the jambs for at least 9 inches on each side. These bars are called " Chimney Bars," and are made from 3/8 to 3/4 inch thick and 2 to 3 inches wide, and are shaped as shown in Fig. 162, with the ends split and turned up and down, one bar being used for each 4 1/2-inch horizontal thickness of the arch. Lintels of stone or of coke breeze concrete or fireclay are also used for spanning chimney openings, and cheap substitutes are occasionally employed for these wrought-iron chimney bars.
The Backs of Chimney Openings should not be less than 9 inches, although 4 1/2-chin backs are used for fireplaces both in internal or external walls; but they are unsatisfactory in the latter position, as the warm gases become chilled and the draught is thereby retarded. The backs of chimney openings in party walls from the hearth up to a height of 12 inches above the mantel should be 9 inches thick at least.
The Jambs on each side of every chimney opening should be at least 9 inches wide.
The Flue Linings of every chimney should be at least 4 1/2 inches thick.
The brickwork above the fireplace is gathered over or contracted above the openings in order to reduce its size to that required for the flue. This is done by projecting the courses as in corbels, the oversailing angles being cut off in good work, as they tend to check the up draught. The gathering over should also be performed in as short a height as possible consistent with good bond in the brickwork, as a large space above the fire is productive of sluggish draught until the fire beneath has thoroughly burnt up.
Brick flues for kitcheners are usually made 14 by 9 inches, while those for ordinary fireplaces are more often 9 by 9 inches. All are rectangular in shape. Circular fireclay tubes 10 inches in diameter (see Fig. 163) are also used for flues, and they possess the advantages that -
They retain the heat, thus making the draught more regular;
They offer less resistance to the passage of the waste products of combustion;
They lessen the risk of fire or of smoke penetrating into adjoining flues;
They are not easily dislodged, as bricks are apt to be by clumsy sweeping;
They are easily fixed in position.
They are, however, too smooth for soot to properly collect on, so that it soon falls on the fire and becomes a nuisance, while the brickwork has to be cut to fit round the pipes and the space between to be thoroughly grouted, which tends to imperfect work.
All flues are inclined, as shown in Fig. 164, with easy curves at the junctions of straight portions, to reduce the risk of down draughts and to bring them away from the fireplaces above. The amount of projection of the bricks in successive courses necessary to produce the required angle of inclination is usually determined as shown in Fig. 164. The distance X is 4 feet 6 inches (i.e. 18 courses), while Y is 4 feet 6 inches; then the projection of each brick will be 54 inches divided by 18, which is equal to 3 inches. Blunders, which can only be remedied by pulling down the brickwork for the distance X, often occur through workmen neglecting to make this simple calculation. The projecting angles of the bricks should be cut away and the receding angles on the opposite side of the flues filled up with mortar. The Withes or partitions between the flues should never be less than 4 1/2 inches. These may be inclined at any angle if soot doors are built into them, but in no other case should they be inclined at a smaller angle than 45 degrees to the horizon. When a flue is inclined at an angle of less than 45 degrees its upper side should be composed of 9-inch brickwork. It is always better to make the inclination of flues at such angles that the offsets in the brickwork need not exceed one-sixth the length of a brick.
Section thro'. B.B.