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246 CHAPTER 17 1960 Guide Table 2 .... Relation of Resistor Voltage to Heat Oefivery Condition Volts Hoot Dotirorod % Electric system, nominal value.................. For design of equipment, at terminals----- Range, as basis for design Minimum.................................................... Maximum.................................................... On secondary distribution system Favorable sone, as to voltage conditions Minimum................................................. Maximum................................................. Tolerable zone Minimum................................................. Maximum................................................. Emergency conditions.................................. 120/240 118/236 110/220 124/248 110/220 125/250 107/214 127/254 90/180 103.3 100 86.8 110.5 86.8 112.0 82.2 116.8 58.3 design of heating system, and internal sources of heat. The NEMA Manual reports that based upon experience with many thousand electric heating installations, the value rec ommended for C is 18.5 unless local experience of a statistical nature has established a more reliable value for the particu lar area concerned. REA Bulletin 142-1 Electric House Heat ing suggests correction factors for degree days to be applied to the NEMA formula when the annual degree days are less than 1800. For block-type electric rates, multiplying the annual kilo watt-hour energy consumption by the cost per kilowatt-hour indicates the annual cost. For demand rates, the assistance of the local utility company should be secured to estimate de mand and energy uses by the month on the particular rate involved. It may be pointed out that for residential systems properly applied with thermostatic control tn each room, the minimum operating demand is about 75 percent of the total calculated load. Voltage Requirements The preferred nominal system voltage at point of electric utilization by equipment, for tingle-phase 3-wire systems nec essary with space heating, is 120/240 volts, as stipulated by standards of the electrical industry (EEI Publication R-6 and NEMA Publication No. 117 issued May 1949). For household heating appliances mentioned in the standards, such as air heaters, water heaters, and cooking ranges, the equipment voltage rating for design is specified as 118/236 volts, and the range of voltages to be used as a basis of design extends from 110/220 minimum to 124/248 maximum. How ever, natural variations exist from time to time in the condi tions at different points in any secondary distribution system and will affect both voltage level and range of fluctuations. Under emergency conditions on electricity supply systems, voltages of the order of 90/180 may be encountered. Heat delivery by resistors with these voltage values, expressed in percent of rated delivery with the normal 118/236 volts (at terminals) is given in Table 2. Voltages stipulated for secondary distribution systems are at point of service entrance to the building; the drop of vol tage in the house supply wiring to terminals of the heating equipment may be 2 to 3 percent, thus reducing heat delivery by some 5 to 8. percentage points below the favorable zone and tolerable zone values included in the last column of Table 2. Accordingly, it is necessary the designers of electric heating in^nationR obtain specific information from the local electric utility company on both existing and anticipated future conditions at the location, before specifying the capac ity and voltage for equipment, wiring, and controls. BIBLIOGRAPHY W. F. Friend: Electric house heating--load characteristics and economics (Midwest Power Conference Proceedings, April 1951). E. E. Parks: Electric house heating (Electrical Engineering, August 1951). Electric House Heating (Aural Electrification Administra tion Bulletin 142-1, December 1957). F. A. Compton: Complete Electric House Heating (Edison Electric Institute Bulletin, May 1950). C. E. Simpson: House heating experience (Electrical World, October 9, 1948). B. H. Martin and T. W. Newberry: Heating by electricity in Tennessee v&Uey area (Heating ana Ventilating, May 1948). H. G. Kelsey: Longview house heating data (Electrical West, September 1945). W. B. Morrison: Electric storage heating serves new Oregon school (Heating, Piping and Air Conditioning, June 1949). R. E. Sinclair: Electric bouse beating load characteristics (Electrical West, September 1950). J. B. Cochran: House heating load characteristics as they affect wiring costs (Electrical World, April 12,1947). H. C. Bender: Heat factor formula to calculate electric house heating (Electrical World, May 12, 1945). W. F. Friend: Modulating and load-limiting controls for electric house heating (American Power Conference Proceed ings, March 1953, and Heating and Ventilating, August 1953, p. 82). Methods developed for built-in radiant heat (Electrical West, December 1948). R. J. Loreoxi and J. F. Schreiber: Performance of electrical system of panel heating with four stages of insulation (Hearing, Piping and Air Conditioning, January 1949). Applications of radiant energy (Illuminating Engineering Society, lighting Handbook, 1952, Section 18). Radiant Glass Heating Panels (National Bureau of Standards, Technical News Bulletin, May 1953). P. R. Achenbach: Radiant glass heating panels (Hearing and Ventilating, January 1953, p. 83). EEI-NEMA Preferred Voltage Ratings for A-C Systems and Equipment (Edison Electric Institute Publication No. R-6, May 1949, and National Electrical Manufacturers Association Pubiscation No. 11, May 1949). Standard Handbook for Electrical Engineers (McGraw-Hill Book Co, New York, 1952). R. E. Sinclair: Short method for estimating electric house heating load (Air Conditioning, Hearing and Ventilating, Janu ary 1955). R. L. Boyd: Heat schools electrically? (Heating, Piping and Air Conditioning, December 1956). NEMA Manual for Electric House Hearing (National Elec trical Manufacturers Association, June 1957). J. C. Beckett: Cost comparison: resistance space heating vs fuel-fired systems (Electrical Construction and Maintenance, October 1956). Space heating: what happens in the 6,000 degree-day zone (Electrical World, March 19, 1956). W. R. New: Serving the all-electric home (Electrical World, March 19, 1956). R. L. Boyd: It's here...the all electric school (Electrical Construction and Maintenance, February 1957). ~ Handbook of Electrical Applications (Edison Electric Insti tute). W. J. Novak: Electricity and combustible fuels (Electrical Construction and Maintenance, April 1957). CHAPTER 18 WARM AIR HEATING SYSTEMS Forced Warm Air Systems/ Air Distribution/ Supply Outlets and Return Grilles/ Duct Construction; Simplified Methods of Design for Perimeter Systems of loop, Radial, and Extended-Plenum Types/ Return Duct System for Perimeter Installations,- Inside Wall Delivery Systems/ Design of large Systems/ Automatic Controls; Ad/ustment of System; Warm Air Ceiling Panel Systems/ Summer Operation/ Gravity Warm Air Systems ARM air heating systems may be conveniently di units and the function and selection of the various parts of a Wvided into two classifications depending upon the system are discussed in Chapter 35 and other publications.1 manner in which the motive power for circulating the warm air is supplied. In gravity systems, the motive bead is due AIR DISTRIBUTION to the difference in weight between the heated air leaving the top of the furnace waging and the cooler return air en tering tiie bottom of the casing. In a forced warm air-heating system, all or part of the motive head is supplied by a fan.' Although a great many gravity warm air heating systems are in use, only a limited number of new gravity installations are now being made. Emphasis will therefore be placed upon the design and installation of forced warm air heating sys tems. The conditions of comfort obtained in a room are influ enced greatly by the type of supply outlet used, and the locations of the supply outlets and return grilles. In general it has been found that changes in the type, air velocity, and location of the supply outlet affect the room conditions much more than the changes in the location of the return grilles. Although the principles of air distribution which are dis cussed in Chapter 20 apply in the design of warm air heating systems, simplified methods of selecting outlet sice and PORCH) WARM AIR SYSTEMS location are usually used. Three methods of locating outlets and grilles are in common use. One method is to locate all In forced warm air heating systems, the air circulation is effected by motor-driven centrifugal fans, commonly referred to as blowers. The advantages of forced air systems are: supply outlets in or near the outride wall, usually beneath windows, and to use outlet grilles or diffusers which deliver the air upward in a fan-shaped pattern to blanket the cold area. This arrangement causes mixing of the warm supply 1. The furnace may be placed in any part of the structure. air with the cool air from the bigh-heat-loss area and with 2. Distribution duets can be made small enough to be in conspicuous and out of the way, or be completely concealed from view where desired. 3. Circulation of air is positive, and in a properly designed system, can be controlled in such a way as to give a comfortably uniform temperature distribution. 4. Humidity control is readily attained. 5. The air may be cleaned by filters or other means. 6. If properly designed or-suitably adapted, the same air distribution system can be used for summer cooling as for winter heating. the cold air from infiltration, to prevent drafts. This method of introducing warm air into a structure is called perimeter heating. A second method is to locate the supply openings on the inride wall, near the floor or near the ceiling, and locating the return openings near the greatest outride exposure. A third method is to locate the supply outlets in the ceiling, and the return openings near the greatest outride exposure. Any of these methods of introducing the supply air can be used for summer air conditioning. In any case, the warm air registers should be so located and 7. The use of the fan permits flexibility in the location of supply and return grilles as required to obtain proper distribu tion of air for comfort. 8. Controlled quantities of ventilation air may be drawn into the system and conditioned before it is introduced into occupied portions of the structure. Recirculated air can be treated as necessary to obtain the desired quality. so designed that the air stream never discharges directly against people at rest. Tests* in Warm Air Research Resi dences No. 1 and No. 2 at the University of Illinois, have indicated that nearly continuous blower operation gave bet ter results than intermittent operation. This type of opera tion is more commonly achieved by suitable adjustments of Warm air heating systems are equally adaptable for use in residential and nan-residential structures. Although most air quantity and fan switch settings than by continuous operation of the blower.* commonly found in one and two family residences, recent developments in equipment and system design have con Supply Outlets and Return Grilles tributed to the increased use of warm air heating systems in The type of supply outlet that should be used depends school buildings, apartment houses, and commercial and upon the type of distribution system to which it is applied. industrial structures. Other factors which have contributed With perimeter duct systems, the supply outlets are usu to this trend include the desire to use one distribution system ally located in the floor, in the baseboard, or low in the for both winter and summer air conditioning, and the desire ride-wall and underneath a window. In order to be most ef of owners to turn over to the tenant responsibility for all fective, perimeter diffusers must deliver the air upward and utilities in apartments and commercial buildings. in a fan-shaped pattern so as to blanket the window or cold The* construction features of forced warm air furnace wall with warm air, thus mixing-with and tempering the . 247