This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
In planning chimneys, the points to be considered are the height, and the number, size, and arrangement of the flues. Attention must also be given to the location in respect to valleys, etc. on the roof. To make the chimney "draw" properly, a separate flue should be provided, extending from each fireplace to the top of the chimney. For ordinary stoves and small furnaces, the flues may be 8 in. X 8 in.; but if the furnace is large, it is hotter to make the flue Sin. x 12in., and the same size should be used, when possible, for fireplaces having large grates. (See Fireplaces.) Flues are sometimes only 4 in. wide; but are then easily choked with soot and difficult to clean, so that a flue should not be less than 8 in. wide. Flues should always be lined with some fireproof material; in fact, the building laws of large cities so require. The lining is usually fireclay, tile, or gal-vanized-iron pipe. If the pipe used is round, the space between it and the walls of the chimneys may be utilized for ventilation. The outer walls of chimney flues should be 8 in. thick, if flue linings are not used. Whenever it is necessary to change the direction of the flue, the diversion should be effected by long curves, and not by sharp turns, which retard the passage of smoke. Chimneys should extend above the highest point of the building or of those adjoining; otherwise they are likely to smoke. This may be obviated, when the chimney is not carried high enough, by using a hood having two open sides; but as hoods are unsightly, their use should be avoided, when possible. The top of a chimney should be covered by a stone slab, either solid, to prevent the entrance of rain - the smoke passing out through holes at the sides near the top - or with a hole the size of the flue cut in it, preventing disintegration of the exposed mortar joints.
The general construction of a fireplace is shown in Fig. 14. The projection in the room is called the chimney breast, and its width a is generally 5 ft., which is the standard width for ordinary fireplaces, the return b in this case being 17 in. The height of the fireplace opening at c is about 30 in. from the finished floor line to the springing line of the arch, the rise of which may be 3 or 4 in. The width of the opening between the rough bricks, as d, should be 25 or 26 in., and the depth of the niche e, from 12 to 13 in. These measurements may be adjusted to suit special grates and fittings.
The arrangement of the floor framing requires no explanation, other than that the trimmer joists should be kept 2 in. from the brickwork. A temporary wooden center supports the brick trimmer arch during construction. The space above the arch is leveled up with cement concrete, upon which is laid the slate or tile hearth. With this method of constructing a fireplace, it is almost impossible for the woodwork to catch fire.
The opening above the fireplace, or throat, is gradually contracted to the size of the flue lining, as shown at l and m. Several courses of brick are corbeled out from the back, as at n; on the ledge formed is placed a sliding iron damper. A preferable form of construction consists in the insertion, just below the throat I, of a tilting damper, which may be operated by a key placed in the front of the flreplace.
A good proportion for flues of fireplaces in which it is intended to burn wood or bituminous coal, is to make the sectional area of the flue 1/10 that of the fireplace opening, if the flue is rectangular, and 1/12 if the flue is circular; thus, for a fireplace which is 30 in. wide X 30 in. high, or 900 sq. in. in area, the area of a rectangular flue would be 90 sq. in., which is nearly equivalent to an 8" X 12" flue; for a circular flue it would be 75 sq. in., which is equivalent to a 10" flue. When anthracite coal is to be used in the fireplace grates, the flue an m may be made 1/15 that of the fireplace opening for the rectangular form, and 1/18 for the circular.