Document wD9EKX605QE6O399KkgvJ21KD

446 CHAPTER 18 1952 Guide - Table 8. Return Air--Carrying Capacity--Btu Serviced Return Air Com bination No. Duct Dia In. Type A. Btu per Hr Types B' and C Btu' per Hr Type D . Btu per Hr Type E Btu-per Hr Type F Btu per Hr Return Air Com* BINATION No. 3132 33 34 ' 35 36 37 38 10 11,300 9,500 7,800 12 16,300 13,700 : 11,300 5,000 7,200 7,800 11,300 31 32 14 22,200 18,700 15,300 9,800 T5.300 ' 33 16 29,000 24,400 20,000 12,800 20,000 34 18 36,700 30,800 25,300 16,200 25,300 20 45,300 38,000 31,300 20,000 31,300 35 36 22 54,800 46,000 37,800 24,100 37,800 24 65,200 ,54,800 45,000 28,700 45,000 37 38 Design Procedure The steps to be taken in designing a gravity warm-air duct system are: 1. Calculate, the heat loss from each room as explained in Chapters 9, 10 and 11. 2. Prepare a layout showing (e) furnace, (6) chimney connection, (c) warm air registers (whether floor, baseboard or wall), (d) return air grilles. 3. Indicate on each warm-air leader (using symbols shown in Fig. 6): (a) whether the room to be heated is on first or second story; (6) the approximate length of leader pipe in the basement; (c) the number of right angle elbows and equivalent elbows (Table 5) required, including the elbow at the boot connection (see Fig. 2); (d) whether the register is to be, located in the floor, in the baseboard, or in the wall. Up to 20' Same as Type E except that col* tar and.shoe are same as for Type A. Type F Note: For Types C, D, E, and F return-air duct systems, reduce the carrying capacities shown in Table 8 by 1 percent for each 4 ft additional length in the horizontal run. Fig. 5. Typical Arrangements of Return-Air Duct Systems ^Gravity. Warm Air Systems 447 4. Show the number and proposed locations of return-air grilles and the type of return-air system (see Fig. 5). 5.. From Table 6,.for.first story, or from Table 7 for second story, select,thecom- bination number for the warm air system which will supply the heat required'to each room, with' the number, of elbows and length of leader pipe previously deter mined.' Then, using the combination, number as found, reid directly in Tables 1, 2, or 3 the leader, stack, and register sizes required. .. . 6. From Table 8 select the combination number for the return-air system to cor respond with the Btu serviced and the type of return-air system. Then from Table 4 select the duct!and grille sizes, etc., corresponding to the same combination hiun- ber. ' . 7. Select a furnace having a register delivery, in Btu per hour, equal to the total heat loss from the structure. Standard work sheets to facilitate design according to the above recom mended procedure, are available from N.WA.H. & A.C.A. Fia. 6. Typical Basement Line Drawing Design Examples Examples 1 and 2 will illustrate the use of the tables in selecting warm-air and return-air system sizes. - Example 1: For a room which has a heat loss of 22,500 Btu per hr, select the size of first story warm-air system. There are three elbows, and the leader is approxi mately 10 ft long. Solution: Since 22,500 Btu is beyond-the capacities shown in Table.6, it is neces sary to select two units of 11,250 each. From Table 6 in 10 ft leader column, and insection for three elbows, find 11,400 as nearest capacity which corresponds to Com bination Number 4 in first column. Refer to Combination Number 4 in Table 1 and find that the leader should be 12 in. in diameter, and should be used with a 12 X 14 in. floor register or a 13 X 11 in. baseboard register with a 5} in. extension. Example S: What is the size of a return-air system of Type D which is to serv ice 35,000 Btu per hr? Solution: From Table 8 find Combination Number 37 which will service 37,800 Btu per hr. Refer to Table 4 to find that Combination Number 37 will require a 22-in. diameter duct, a shoe area of 420 sq in., a metal grille 18 X 30 in., a duct42 X 10 in. or 36 X 12 in. If joist lining is UBed the minimum depth should be 15 in. for two 2-joist Bpaces 14 in. wide, or 10 in. for three joist spaces.