Document jg54rvMq39QX6wrpwML46okOp

398 CHAPTER 21 1948 Guide In a gravity circulating system, the. ratio of stack to leader area is quite important, although little is gained by providing wall stacks with areas in excess of 75 per cent of their connected leader pipe area. In most cases a 334 in. X 12 in. stack is the largest which can be installed' in normal wall construction. Hence, any room having a heat loss much in excess of 9000 Btu per hour, will require two or more stacks, or one oversized stack built into a 6 in. studding space, providing the design register temperature is to be retained at the value of 175 F as recommended. Fig. 1. A Sectional View of a Typical Plant Showing Good Installation Practice3 A. House chimney, no bends nor offsets. N. Sleeve with air space around leader where B. Top of chimney at least 2 ft above ridge of passing through wall. roof. O. Dampers in all leaders, except one. t C. 1 Flue lining, fireclay. P* Transition fittings. D. All joints air tight. Q. Rectangular wall stack. E. At least 8 in. brick. R. Baseboard register. p. No other connection besidethat to furnace. S. Distribute pipes equally around bonnet. G. Cleanout frame and door, airtight. T. Floor register. H. Smoke pipe, end flush with inner surface of flue. U. Return air face. I. Draft door. . V. Panning under joist. J. Use flue thimble. W. Transition collar. K. Casing body. X. Round return pipe- L. Casing hood or bonnet, top of all leader collars Y. Transition shoe. on same level. . Z. Top of shoe at casing not above grate level. M. Round leader, pitch 1 in. per foot. aFrom N.WA.H.&A.C. Assn. Standard Code Application Manual. Registers used for discharging warm air into rooms should have a net area not less than the area of the leader pipe to which the register is attached. First story registers should be connected through boot and register box extensions having areas at least equal to leader areas. Upper story registers should be of the same width as the wall stack, and shouldbe placed either in the baseboard or sidewall, preferably without offsets. 1 Gravity Warm Air Systems 399 First story registers may be of the baseboard or floor type, with the former location preferred. High sidewall registers in gravity systems deliver more warm air into the room than do baseboard registers, but most of the additional air merely results in high temperatures at the ceiling. RETURN AIR GRILLES, DUCTS, AND SHOE CONNECTIONS The placement and number of return grilles will depend upon the size, details, and exposure of the house. Small compactly built houses may be adequately served by a single return grille effectively placed in the central hall. It is usually desirable to have two or more returns, provided that in two-story residences one return is placed to effectively receive the return air at the foot of the stairs. A return air connection must be carried to any room whose floor level is below that of adjacent rooms. The return air grilles should have free areas at least equal to the ducts to which they connect and should be installed in the floor, or in the base board with the top edge of the grille not more than about 14 in. above the floor line. Frictional resistance in the return air system is as detrimental as is resistance in the warm-air system, so that care should be exercised in locating return air grilles which require long return ducts. Where a divided system of two or more returns is used, the grilles must be placed to serve the maximum area of cold wall or windows. Thus, in rooms having only small windows the grille can be brought as close to the furnace as possible, but if the room has large window exposure the grille should be located near the exposure. The frictional resistance of the long ducts used in parallel with short return ducts must be reduced to compensate for the length. Return ducts from upstairs rooms may be 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. The ducts through which air is returned to the furnace should be designed to minimize resistance to air flow. They should be of ample area, in excess of the total area of warm-air pipes, and should be stream lined. Horizontal ducts should pitch at least 34 in. per foot downward toward the furnace, avoiding fittings which would require lifting of the return air after the duct has passed under some obstacle. 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 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