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HEATING VENTILATING AIR CONDITIONING GUIDE 1941 Electric steam boilers are useful in industrial plants which require limited amounts of steam for local processes, and also for sterilizers, jacketed vessels and pressing machines which need a ready supply of steam. It sometimes is economical to shut down the main plant fuel burning boilers when the heating season ends, and to supply steam for summer needs with small electric steam boilers located close to the operation. In general, electric steam heating is confined to auxiliary or other limited applications. If the heating system is designed to use electricity exclusively, steam generating or distributing equipment is superfluous. ELECTRIC HOT WATER HEATING .Electric water heating, using an electric boiler in place of a fuel burning boiler, like electric steam heating, is generally confined to auxiliary or CHAPTER 43. ELECTRIC HEATING HEATING DOMESTIC WATER BY ELECTRICITY1 Electric water heaters of the automatic storage type for domestic hot water supply are simple and reliable. In many sections of the country low electric rates have been established by the electric utilities to secure this load. In some localities, electric rate schedules divide the current used for water heating into two classifications, regular and off-peak. A time switch automatically limits use of the off-peak heating element to the hours of off-peak load, while the regular heating element is a stand-by at all times. Storage of this two-element type of water heater is larger than average to carry over the periods when the off-peak element is timed out, without too frequent demands on the regular heating element which takes the higher domestic lighting service rate. Some utilities now offer Fig. 5. Diagrammatic Arrangement of an Electrode Boiler other limited applications. The use of insulated water storage tanks, in which to store heat generated by electricity during off-peak hours at extremely low rates, is a development which has some special applications. In this system of heating, the primary storage tank is simply a large, well-insulated, pressure type steel tank, equipped with electric heating elements and automatic time switches, which also have automatic limit controls for temperature and pressure. The heating system installed in the building may be of any standard individual radiator or fan-served indirect type or with provisions for the heating and humidification phases of an air conditioning system. A system of this kind requires very careful design to avoid excessive overall radiation losses during periods of low heat demand/ It is also important to provide for sudden.changes in heat demand. A typical hot water heating boiler is illustrated in Fig. 4. 754 Fig. 6. Piping Arrangement for Connecting Electric Water Heater to Fire-Box Coil Fig. 7. Domestic Hot Water Heater for Off-Peak Service a schedule which, beyond a stipulated minimum, lowers the rate for all electric service if an electric water heater is installed. Competition with other fuels, especially gas, seems to be the major controlling factor in the use of. electricity. The first cost of electric storage heaters is also greater than for gas, owing to' the need for larger tank storage due to off-peak service and slower recuperating capacity. In residential work, to effect a saving in the cost of operation, it is sometimes desirable to use a furnace coil or indirect heater in connection with an electric water heater. In this case it is important to make the proper connections in order to benefit by any heat obtained from the furnace and at the same time to prevent dangerous overheating. The proper piping connections are shown in Fig. 6, and in this case the electric heater will only furnish heat when insufficient heat is supplied from the furnace. This arrangement has a further advantage in the summertime in that the bare tank through which the cold water passes on its way to 'Test Results of Electric WaterHeaters, by C. G. Hillier (A.S.H.V.E. Journal Section, Healing, Piping end Air Conditioning, November, 1936, p. 632). Fourteenth Range and Water Heater Survey CElectric Light and Power, August, 1940). 755