Flue

The area of lined flues should be a twelfth or more of the fireplace

opening, provided the chimney is at least 22 feet in height, measured

from the hearth. If the flue is shorter than 22 feet or if it is unlined,

its area should be made a tenth or more of the fireplace opening. The

fireplace shown in figure 34 has an opening of 7.5 square feet, or

approximately 1,080 square inches, and needs a flue area of approximately

90 square inches; a rectangular flue, 8% by 18 inches, outside dimensions,

or a round flue with a 12-inch inside diameter might be used, as

these are the nearest commercial sizes of lining (table 2). It is seldom

possible to obtain lining having exactly the required area, but the inside

area should never be less than that prescribed above. A 13- by

13-inch flue was selected for convenience when combining with the

other flues. If the flue is built of brick and is unlined, its area should

be approximately one-tenth of the fireplace opening, or 108 square

inches. It would probably be made 8 by 16 inches (128 square inches)

because brickwork can be laid to better advantage when the dimensions

of the flue are multiples of 4 inches. The principles of construction

given under Chimneys (p. 7) apply to fireplace flues.

Table 4 is convenient in selecting the proper size of flue or for

determining the size of fireplace opening for an existing flue. The area

of the fireplace opening in square inches is obtained by multiplying the

width, _w_, by the height, _h_, (fig. 34), both measured in inches.