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236 CHAPTER 17 1959 Guide Table 2 .... Relation of Resistor Voltage to Heat Delivery Condition Volts Hoot Detfvervd % Electric system, nominal value.................. For design of equipment, at terminals___ Range, as basis for design Minimum..................................................... Maximum..................................................... On secondary distribution system Favorable zone, 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. Ex perience with many thousands of electric heating installations has shown that a conservative value for C is 18.5. Values of C as low as 12 have been reported on individual installations. It is recommended that a value of C of 18.5 be used unless local experience of a statistical nature has established a more re liable value for the particular area concerned in the estimate. 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 residences with thermo static control in each room generally establish a maximum of 75 percent of the total calculated heat load as the highest demand requirement for heating. Voltage Requirements The preferred nominal system voltage at point of electric utilization by equipment, for single-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 installations 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 bouse heating--load characteristics and economics (Midwest Power Conference Proceedings, April 1951). E. E. Parks: Electric house heating (Electrical Engineering, August 1951). Electric House Heating (Rural 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 valley area (Heating and 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 house 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. Lorenzi and J. F. Schreiber: Performance of electrical system of panel heating with four stages of insulation (Heating, Piping and Air Conditioning, January 1949). L. N. Roberson: Radiant heating by electricity (Healing and Ventilating, September 1946, p. 89). j R. S. Tice: Low-voltage high-current radiant heat (Electrical West, December 1947). .' Applications of radiant energy (Illuminating Engineering Society, Lighting Handbook, 1952, 8ection 18). iladiant Glass Heating Panels (National Bureau of Standards, Technical News Bulletin, May 1953). P. R. Achenbach: Radiant glas heating panels (Heating and Ventilating, January 1953, p. 83). BE1-NEMA Preferred Voltage Ratings for A-C Systems and Equipment (Edison Electric Institute Publication No. R-6, May 1949, and National Electrical Manufacturers Association Publi cation 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, Heating and Ventilating, Janu ary 1955). R. L. Boyd: Heat schools electrically? (Heating, Piping and Air Conditioning, December 1956). NEMA Manual for Electric House Heating (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 Worm 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; Adjustment of System; Warm Air Ceiling Panel Systems; Summer Operation; Gravity Warm Air Systems ARM air heating systems may be conveniently di posed walls and glass.-A second method is to locate the sup Wvided into two classifications depending upon the ply openings near the floor,.or high in the side wall, on the manner in which the motive power for circulating the warinmside wall, and the return openings near the greatest outside air is supplied. In gravity systems, the motive head is due exposure. A third method is to locate all supply openings to the difference in weight between the heated air leaving around the outside wail, near the source of the greatest heat the top of the furnace casing and the cooler return air en loss, usually beneath the windows, and to use grilles de tering the bottom of the casing. In a forced warm air heating signed to blanket the cold area. This causes mixing of the system, all or part of the motive head is supplied by a fan. warm air delivered with the cool air from the heat loss area Although' a great many gravity warm air heating systems and the cold air from infiltration, thus effectively prevent are in use, only a limited number of new gravity installations' ing drafts. It has been called perimeter heating. are now being made. Emphasis will therefore be placed upon In any case, the warm air registers should be so located and the design and installation of forced warm air heating sys so designed that the air stream never discharges directly tems. against people at rest. Tests* in Warm Air Research Resi FORCED WARM AIR SYSTEMS dences No. 1 and No. 2 at the University of Illinois, have indicated that nearly continuous blower operation gave bet 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: ter results than intermittent operation. This type, of^opera tion is more commonly achieved by suitable adjustments of air quantity and. fan switch settings. than by continuous, operation of the blower* . .. 1. The furnace may be placed in any part of the structure. - 2. Distribution ducts can be made email 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. Supply Outlets and Return Grilles The type of supply outlet that should be used depends upon the type of distribution system to which it is applied. With perimeter duct systems, the supply outlets are usu ally located in the floor, in the baseboard, or low in the 4. Humidity control is readily attained. side-wall and underneath a window. In order to be most ef 5. The'air may be cleaned by filters or other means. fective, perimeter diffusers must deliver the air upward and 6. If properly designed or suitably adapted, the same air distribution system can be used for summer cooling as for winter heating. 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. in a fan-shaped pattern so as to blanket the window or cold wall with warm air, thus mixing with and tempering the cold air which usually descends along a cold surface. Fur thermore, since delivery of the air outward into the room, rather than upward, is likely to impinge upon the occupants 8. Ventilation air may be positively introduced and con ditioned. . it should be avoided. With certain types of perimeter dif fusers, the air velocity at the diffuser face is limited only by The construction features of forced warm air furnace units and the function and selection of the various parts of a system are discussed in Chapter 35 and other publications.1 the pressure available at the diffuser and the noise charac teristics of the outlet. Tests with inside wall supply systems conducted in Warm Air Heating Research Residence No. 1 have indicated that AIR DISTRIBUTION comparable results are obtainable with either high ride-wall or baseboard registers, if proper registers and air velocities The conditions of comfort obtained in a room are influ are selected. enced greatly by the type of supply outlet used, and the Baseboard registers should be of a deflecting-diffuser type locations of the supply outlets and return grilles. In general that will direct the air downward toward the floor and dif it has been found that changes in the type, air velocity, and fuse it at the 6ame.time.' For baseboard registers, air veloci location of the supply outlet affect the room conditions much ties over 500 fpm should be avoided as they may cause dis more than the changes in the location of the return grilles. comfort. Three methods of locating outlets and grilles are in common High ride-wall registers should be of a type that will de use. One method is to locate the supply register near the liver the air horizontally or in a slightly downward direc floor so that the warm air from the register blankets a cold tion, and should be so located as to avoid impingement of wall, and mixes with the cold air descending from the ex air on ceiling or wall. Directional flow diffusing type regis- 237