tag="{http://www.w3.org/1999/xhtml}a">fig. 2). Cracked and leaky flues not only are inefficient, destroying the draft as well as permitting smoke and gases to pass into adjacent rooms, but are a dangerous fire hazard. The chimney as known today was developed about 600 years ago. Experience has shown that the satisfactory performance of a chimney flue is determined by its size, direction, shape, height, tightness, and smoothness.

Draft

The draft of a chimney is the current of air created by the difference in pressure resulting from variation in weight between the relatively hot gases in the flue and the cooler outside air. The strength or intensity of the draft depends, for the most part, on the height of the chimney, and the temperature difference between the chimney gases and the outside atmosphere. The draft is not so good in summer as in winter because the difference in temperature between the outside air and the gases in the flue is less.

A very common error in chimney design is failure to distinguish between the size of the flue required for free passage of the volume of smoke from a given amount of fuel and that which, with proper height, will produce the required draft. A chimney may be high enough (fig. 3), yet have an area too small to expel the volume of smoke; or the size may be ample (fig. 4) but the height not great enough to produce a strong draft. Either fault or a combination of the two will result in unsatisfactory service.

Flue Sites

The dimensions of a flue for adequate draft depend principally on the grate area and type of heating plant[1] and on the kind of fuel to be burned, both of which should be determined before construction is begun. If a chimney is found to be inadequate the only method of improving it, short of reconstruction, is to increase its height. This is not always effective and is often impracticable.

Table 1 gives the sizes of fire-clay flue linings ordinarily provided for boilers, furnaces, stoves, or convection heaters burning soft coal. These sizes have proved satisfactory for average flat-grate furnaces under normal conditions. Manufacturers of heating equipment usually specify certain requirements in chimney construction and will not guarantee the performance of their heaters unless these requirements are met. Therefore their recommendations should be followed when differing materially from the dimensions given in this bulletin.

Height of Chimney

A chimney should extend at least 3 feet above flat roofs and 2 feet above the ridge of peak roofs. Where chimneys cannot be built high enough above the ridge to prevent trouble from eddies caused by wind being deflected from the roof, a hood may be provided with the open ends parallel to the ridge. Eddies which force air down the flues may be caused by building the chimney too near trees (fig. 5, B) or a higher structure (fig. 6).

Figure 2.—Heavy masonry chimneys of this type are still being built in rural areas. The thick walls, with unlined flues, are in good condition after 75 years of continuous use.

Table 1.—Sizes of flue linings and heights of chimneys recommended for flat-grate furnaces burning soft coal[A]