[Illustration: Figure 39.--Diagram showing front view and cross section of an entire chimney such as is commonly built to serve a furnace, fireplace, and kitchen stove. Two sets of dimensions are given, those in rectangles refer to the approximate sizes of the voids or openings; the others refer to the outside dimensions of the brickwork. These are used in estimating the number of bricks in a chimney. The letters _A_-_F_ indicate sections used in estimating the quantities of brick required (See p. 44.)] 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. COST ESTIMATE A convenient method for estimating the number of bricks in a chimney is to calculate the volume of the various sections which differ in outside dimensions and then subtract the voids or cavities resulting from ash-pits, fireplace, and flues. This will be the total cubic feet of brickwork which, when multiplied by 22.5, is converted to number of bricks. For convenience, inches as indicated in figure 39 have been converted to decimals of a foot.[6] [6] Inches and fractions of an inch are converted to feet and decimals by multiplying by 0.0833; thus 2 X / inches × O.0833 equals 0.208 feet. Table 4.--_Sizes of fireplace flue linings_[D] Outside dimensions Inside Area of of standard diameter of fireplace rectangular standard round opening flue lining flue lining --------- ----------- -------------- _Square inches_ _Inches_ _Inches_