Flue Lining
The flues of chimneys should not start from the bottom of the foundation but only about a foot below the first smoke pipe openings, and should be lined with terra cotta flue lining their entire height. Care should be taken in setting flue linings to be sure that the joints are well cemented and, at the same time, that all spaces between the lining and brickwork are tightly filled with mortar. Any openings in the joints of the tile lining, or even of the brickwork, not only check the draft but are a fire menace. Cement plaster should not be substituted for the flue lining as it is likely to crack and fall off, thus leaving the flue in a dangerous condition. However, where flue linings are not available, a strong smooth cement plaster may be used, in which case the chimney wall should be at least 8 inches thick.
Modern heating plants necessitate accurate construction of chimneys, and most manufacturers of heating apparatus nowadays recommend the area and height of the flue necessary for their installations. The following table will prove useful in considering the question of heating plant or fireplace, by showing the dimensions of flue linings to be ordered when the required area is ascertained.
Table of Commercial Flue Linings
| Outside Dimensions | Actual Inside Area |
| 81/2" × 81/2" | 52 sq. in. |
| 81/2" × 13 " | 80 " " |
| 13 " × 13 " | 126 " " |
| 13 " × 18 " | 169 " " |
| 18 " × 18 " | 240 " " |
| Fig. 18. Chimney Withes | Fig. 19. Chimney Offset |
Offsets
Where two or more flues are contained in one chimney, they should always be separated by a brick partition 4 inches thick, called a withe, and bonded to the outside brickwork as shown in [Fig. 18]. Chimneys should run as straight as possible from bottom to top, in order to secure better draft and facilitate cleaning. If, however, offsets are necessary from one story to another, they should be very gradual, never less than at an angle of 30° from the vertical. If abrupt offsets occur in flues, soot will soon be deposited, choking the flue and making cleaning almost impossible ([Fig. 19]). Care should be taken while the chimney is building that the bottom does not become filled with mortar or brick bats. At the bottom of the furnace flue in the basement, an iron cleanout door should be provided as a convenience for removing soot.
Chimneys erected on the interior of a building are apt to be more efficient because the warm air surrounding them facilitates the draft, while those located on the exterior naturally are somewhat affected by the cool air on the outside.