Document K6v2gNYBMdBz1Vn9kaNEOYwa0

392 CHAPTER 21 1946 Guide necessary in spaces which are closed off from the rest of the house or which have much outdoor exposure. Return grilles on different floor levels should not be connected to the same vertical return duct. Ducts returning air to the furnace should avoid heat sources which tend to reheat the return air. If the duct must be run over the top of the furnace, or above the vent pipe from the furnace, insulation should be interposed between the heat source and the duct. Circulation of air is facilitated if the air can slide down a pipe inclined at approximately 45 deg and into a furnace shoe connection having a cross-sectional area equal to that of the pipe. The top of the return shoe should enter the casing below the level of the grate in the case of a coal Up to 20' - Same as Type E except that col* !ar and shoe are same as for Type A. Type 0 Type E Type F Note: For Types C, D, E, and F return-air duct systems, reduce the carrying capacities shown in Table 8 by 1 per cent for each 4 ft additional length in the horizontal run. Fig. 3. Typical Arrangements of Return-Air Duct Systems furnace, and not more than 14 in. above the floor in the case of oil or gas furnaces. In order to accomplish this the shoe is made wide. OUTLINE OF DESIGN PROCEDURE The data underlying the design procedure are given in detail in a circular2 issued by the University of Illinois. In this procedure the design of the warm-air duct system is considered as an entire unit, so that for a given heat loss the sizes of leaders, stacks, boots, stackheads, and registers are all correlated. Similarly in the case of return ducts, the selection specifies a complete unit consisting of return grille, return duct, and shoe connection. Recommended Standard Sizes For the purpose of simplification and standardization, selected com binations of commercial sizes of warm air pipes, return air pipes, ducts, grilles, fittings, and registers arc designated as Combination Numbers. The numbers assigned and the combinations selected as standard are listed in the following tables 3. Gravity Warm Air Systems 393 Table 1--Combination Numbers 1 to 5--First Story Warm Air Ducts and Registers. Table 2--Combination Numbers 11 to 16--Second Story Warm Air Ducts, Single Wall Stacks, Fittings, and Registers. Table 3--Combination Numbers 21 to 24--Second Story Warm Air Ducts, Double Wall Stacks, Fittings, and Registers. Table 4--Combination Numbers 31 to 38--Return Air Ducts, Fittings, and Grilles. The selected types of boots are shown in Fig. 2 and their resistances expressed in equivalent elbows are shown in Table 5. It is essential that free areas be maintained throughout fittings. The selected types of return air ducts and fittings are shown in Fig. 3. Table 1. First Story Warm-Air Ducts3 Combination No. . Leader Pipe Diameter, In. 18 29 3 10 4 12 5 14 Floor 8 x 10 9x 12 10 x 12 12 x 14 14 x 16 Register Size. In. Baseboard Size 10 x 8 12 x 8 12 x 9 13 x 11 -- - Extension 2K 2J4 . 5H `When the calculations indicate a requirement for a given room greater than Combination No. 4, two or more smaller units totalling the required capacity are recommended. Table 2. Second Story Warm-Air Ducts--Single Wall Stacks and Fittings Combi nation No.. Leader Pipe Diameter. . In. * Stack1* Size In. Floor n8 10 x 3Ji 8x 10 12 9 12 x 3 9 x 12 14 10 14 x 3J 10 x 12 15 12 12 x 5M 16 12 14 x -- Register Size, In. Baseboard Size Extension 10 x 8 12 x 8 12 x 8 12 x 9 13 x 11 2J4 2J4 2K 3)i 5K Sidewall 10x8 12 x 8 12 x 8 bRecommended stack sizes. Tables may also be applied to 3 in. and 3} in. stack depths. Table 3. Second Story Warm-Air Ducts--Double Wall Stacks and Fittings Combi nation No. Leader Pipe Diameter, In. Sta^k Size, In. Internal. External Floor Register Size, In. Baseboard Size Extension Sidewall 21 8 2HC * 10 3 * ioys 8x 10 10 x 8 . 2M 10-x 8 22 8 3 x 10 3y% xio% 8 x 10 10x8 2M - 10 x 8 %23 9 2W- x 12 3 y? x 12^ 9 x 12 12x8 2M 12 x 8 24 9 3 x 12 3 Xl2% 9 x 12 12x8 2M 12 x 8 `Commercial sizes vary in. from values shown.