Fireplaces and Chimneys

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

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]

Grate area (Sq. ft.)	Nominal size of flue lining								Height of chimney top above grate at elevation indicated
	Round (inside diameter) at elevation indicated				Rectangular (outside dimensions) at elevation indicated
	Sea Level	2,000 feet	4,000 feet	6,000 feet	Sea Level	2,000 ft	4,000 ft	6,000 ft	Sea Level	2,000 feet	4,000 feet	6,000 feet
	In.	In.	In.	In.	In.	In.	In.	In.	Ft.	Ft.	Ft.	Ft.
1	8	8	8	10	8½ by 8½	8½ by 8½	8½ by 8½	8½ by 13	2	26	32	36
2	10	10	10	10	8½ by 13	8½ by 13	8½ by 13	8½ by 13	24	29	35	41
3	10	10	12	12	8½ by 13	8½ by 13	13 by 13	13 by 13	26	33	41	49
4	12	12	12	12	13 by 13	13 by 13	13 by 13	13 by 13	30	37	45	49
5	12	12	15	15	13 by 13	13 by 13	13 by 18	18 by 18	32	37	43	52
6	15	18	18	18	18 by 18	18 by 18	20 by 20	20 by 20	30	37	47	56
7	18	18	18	18	20 by 20	20 by 20	20 by 20	20 by 20	32	41	49	64
8	18	18	18	18	20 by 20	20 by 20	20 by 20	20 by 20	35	42	56	10

[A] If anthracite is to be burned the area of the flue cross section may be reduced about 25 percent.

The ratings given are based on comparatively smooth lined flues with no offsets greater than 30° with the vertical.

The smallest sizes of fuels require excessive drafts and may necessitate taller chimneys.

Flue heights and sizes are based upon approximately the several altitudes indicated; it is sufficiently accurate to use the column giving the altitude nearest to that of the particular problem.

When 2 or 3 appliances are connected to the same flue their total grate area may be reduced 15 percent. The method of determining the proper flue size for an altitude of 2,000 feet, when 1 appliance with a grate area of 3 square feet and another with an area of 1.5 square feet are attached to the same flue, is shown by the following example:

Add the 2 grate areas, 3 + 1.5 = 4.5 square feet. Reduce this total area by 15 percent. Thus, 4.5 - 0.68 = 3.8 square feet is the required area. Use the nearest whole number, 4. From the table it is seen that for a grate area of 4 square feet at an elevation of 2,000 feet either a 12-inch (inside diameter) round flue or a 13- by 13-inch (outside dimensions) rectangular flue 37 feet high is required.

Figure 3.—This tall chimney produced good draft for the kitchen range, but the flue was too small for a furnace. When the house was remodeled, its appearance was greatly improved by building the chimney inside.