_Outside Dimensions_ _Actual Inside Area_
8-1/2" × 8-1/2" 52 sq. in.
8-1/2" × 13 " 80 " "
13 " × 13 " 126 " "
13 " × 18 " 169 " "
18 " × 18 " 240 " "
[Illustration: Fig. 18. Chimney Withes]
[Sidenote: 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.
[Illustration: Fig. 19. Chimney Offset]
[Illustration: Fig. 20 Fig. 21 Outside Angle Corners]