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. [Illustration: 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.] [Illustration: Figure 4.--Short chimneys are frequently provided for low bungalows, for architectural reasons. This flue is ample in size but not high enough for use with a stove. The stone masonry has been laid to harmonize with the rustic surroundings.] Frequently metal-pipe extensions are provided to increase the height of a flue on account of the low cost and ease of installation, but these must be securely anchored against wind and have the same area as the flue. Metal extensions are likely to rust in a short time. They are available with a metal cowl or top that turns with the wind to prevent air blowing down the flue. Terra-cotta chimney pots or extensions are more durable and attractive. A chimney located entirely inside a building has better draft because the masonry retains heat longer when protected from cold outside air. [Illustration: Figure 5.--Two pleasingly designed fireplace chimneys that fit into their surroundings. _A_, This chimney stands in the clear and should provide a good draft; _B_, a chimney under overhanging trees is likely to backdraft. Contrast the appearance of these two chimneys with that of figure 3.] [Illustration: Figure 6.--Several extensions were necessary before this chimney would draw properly on account of the wind deflected from the nearby wall.] [Illustration: Figure 7.--_A_, A good foundation extending below the soil affected by frost. This chimney is well protected from ground moisture by the concrete carried above the surface; _B_, an insecure foundation. Supporting a chimney in this manner is a dangerous practice.] CONSTRUCTION