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432 CHAPTER 20 - 1949'Guide . Since values of 17. are not available for oil or coal, an assumed seasonal efficiency Umust be used.. Selection of a value for this E must be made with caution, for its .use implies a meaning not commonly associated with the word efficiency and consequently is frequently misleading. The input of heat to a building consists not only of the energy in the fuel but that from occupants, the sun, appliances, processes, and all other sources. In many cases these make up, over a period, an important per centage of the total heat required, and if they are not taken into account a calculation of efficiency can show a figure over 100 per cent. For this and other reasons the actual, seasonal efficiency is a difficult thing to determine. Published data are widely scattered and -insufficient. From the available published material it is found that the seasonal efficiency varies over a wide range, depending on the fuel used, and it varies widely even for a given fuel. For example, in a recent survey of 30 houses in one locality there was found a variation of from 45 to 75 per cent in the utiliza tion efficiency depending on the fuel. REFERENCES - Graphical Method of Calculating Heat Losses, by Paul D. Close (A.S.H.V.E- Transactions, Vol. 49,1943, p. 345). Standard Gravity Code for the Design and Installation of Gravity Warin Air" Heating Systems (11th edition), and the Technical Code for the.Design and Installs- ' tion of Mechanical Warm Air Heating Systems (National Warm Air Heating and AirCondition Association). See Industrial Gas Series, House Heating (third edition), published by the Amer ican Gas Association. *. Report of Commercial Relations Committee, Proceedings, National District Heal ing Association, 1932. ` The Heat Requirements of Buildings, by J. H. Walker and G. H. Tuttle (A.S.H. V.E. Transactions, Vol. 41,1935, p. 171). Heat Losses and Efficiencies of Fuels in Residential Heating, by R. A. Sherman and R. C. Cross (A.S.H.V.p. Transactions, Vol. 43, 1937, p.185). CHAPTER 21 GRAVITY WARM AIR SYSTEMS Warm Air Leaders, Stacks, and Registers; Return Air Grilles, Ducts, and Connections; Outline of Design Procedure WARM air heating systems of the gravity type are described in this chapter1. In these systems the motive head' producing flow depends upon the difference in weight between the heated air leaving the top ofthe. casing and .the cooled air entering the bottom of the casing, while in the mechanical type a fan may supply all or part of the motive head. A gravity warm-air furnace heating plant consists of a fuel-burning furnace or heater, enclosed in a casing of sheet metal, which is placed in the basement of the building. The heated air, taken from the top or sides, near the top of the furnace casing, is distributed to the various -rooms of the building through sheet metal warm-air pipes. The warm-air pipes in the basement are known as leaders, and the vertical warm-air pipes which are run in the inside partitions of the building are called stacks.. The heated air is finally discharged into the rooms through registers which are set in register boxes-placed either in the floor or in the side wall, usually, at or near the baseboard. A sectional view of a typical plant showing good installation practice is given in Fig. 1. : The air supply to the furnace is usually taken entirely from inside the building through one or more recirculating ducts, although in some cases an outside air supply duct is provided. WARM AIR LEADERS, STACKS, AND REGISTERS In a gravity circulating warm-air furnace system, the size of the leader pipe to a given room depends upon the length of the leader and the tem perature of the .warm air entering the room at the register. For most successful operation, the furnace should be centrally located with respect to register and stack positions so that the leaders will be of uniform length and as short. as possible, in which case the frictional resistance to air flow and the temperature loss from the ducts will be about the same for all runs. In the Standard Code for Installation of Gravity Warm Air Heating Systems, the design was originally based on the heat carrying capacities per square inch of leader pipe"area with register air temperatures of 175 F. In a recent revision of the entire design procedure, as shown in the section entitled Outline of Design Procedure, the carrying capacities of leader pipes. Save been expressed directly in terms of Btu per hour. In general it is advisable to use two or more leader pipes to rooms re requiring more than the capacity of a 12 in. round pipe. The tops of all sizes of leader -pipes should be cut into the furnace bonnet at .the same elevation, and from this point there should be a uniform upgrade of at least 1 in. per foot of run. Leaders over 12 ft in length, or having a large num ber of elbow fittings should be avoided if possible. In cases where such leaders are necessary, it is recommended that smooth transition fittings - be used, and that duct insulation be applied. Asbestos paper, unless of the corrugated type, should not be considered as insulation. To assist 433