Fireplaces and Chimneys

by Arthur H. Senner; T. A. H. Miller

Where offsets or bends are necessary in lined flues, tight joints can be made by mitering or cutting equally the ends of abutting sections ([fig. 8, B]). This can be done if a cement sack of damp sand is stuffed firmly into the lining and a sharp chisel is tapped with a light hammer along the line where the cut is desired. If the cutting is done after the lining is built into the chimney, the lining may be broken and fall out of place. The hole for the thimble can be cut the same way when a special thimble section is not used.

The linings commonly used are rectangular or round. Rectangular linings are better adapted to brick construction than round linings, but the latter are considered more efficient. The sizes commonly used are indicated in [table 2].

Wall Thickness

Walls of chimneys not more than 30 feet high when lined should be 4 inches thick if of brick and reinforced concrete, 8 inches if of hollow building units, and 12 inches if of stone. Linings may be omitted in chimneys having walls of reinforced concrete at least 6 inches thick or of unreinforced concrete or brick at least 8 inches thick, although lining is desirable in the case of brick construction. Also the outside wall of a chimney exposed to the weather is best made at least 8 inches thick.

In chimneys containing three or more flues, building codes generally require that each group of two flues be separated from the other single flue or group of two flues by brick divisions or withes not less than 3¾ inches wide ([fig. 9]) . Where two flues are grouped without divisions, joints in the linings of adjacent flues are safer if staggered at least 7 inches, and particular care should be taken to have all joints filled with mortar. Individual flues are advisable for fireplaces and heating furnaces or boilers.

Table 2.—Dimensions of commonly used standard commercial flue lining

Rectangular linings[B] Outside dimensions (inches) Cross-sectional area Wall thickness Inside Outside Square inches Square feet Inches 4½ by 8½ 23.6 0.26 ⅝ 4½ by 13 38.2 .41 ⅝ 7½ by 7½ 39.1 .39 ⅝ 8½ by 8½ 52.6 .50 ⅝ 8½ by 13 80.5 .78 ¾ 8½ by 18 109.7 1.10 ⅞ 13 by 13 126.6 1.20 ⅞ 13 by 18 182.8 1.70 ⅞ 18 by 18 248.1 2.30 1⅛ 20 by 20 297.6 2.60 1⅜

Round linings[C] Inside diameter (inches) Cross-sectional area Wall thickness Inside Outside Square inches Square feet Inches 6 28.3 0.29 ⅝ 8 50.3 .49 ¾ 10 78.5 .75 ⅞ 12 113.0 1.07 1 15 176.7 1.62 1⅛ 18 254.4 2.29 1¼ 20 314.1 2.82 1⅜ 22 380.1 3.48 1⅝ 24 452.3 4.05 1⅝ 27 572.5 5.20 2

[B] All rectangular flue lining is 2 feet long.

[C] Round flue lining, 6 to 24 inches in diameter, is 2 feet long; that 27 to 36 inches in diameter is 2½ or 3 feet long.