Document wgOY1x6my3ndEzZzL2egLwZ44

546 CHAPTER 30 1946 Guide of an air conditioning system. A system of this kind requires very careful, design to avoid excessive over-all 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. 2. 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 many 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 Fig. 4. Piping Arrangement foe Connecting Electric Water Heater to Fire-Box Coil Fig. 5. Domestic Hot Water Heater for Off-Peak Service out, without too frequent demands on the regular heating element which takes the, higher domestic lighting service rate. Some utilities now offer a schedule which, beyond a stipulated minimum; lowers the rate for ail 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 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. 4, 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 tie summertime in that the bare tank through which the cold water passes on its way to the electric heater, serves as a tempering tank, absorbing, heat from, the basement air and requiring the use of less energy in the electric heater; A typical domestic hot water heater as shown in Fig. 5 is arranged with Electric Heating '______________________________ . `.:. ___________ 547~ . upper and lower heating elements for the usual type of off-peak heating service. The .lower heating element is under the control of the off-peak time switch. However, the upper heating element is usually connected to the line so that, in case the supply of hot water in the tank becomes exhausted, the top thermostat can turn on the top heater and heat a small . supply of water. The top heater will not heat the water in the tank below its location, but when the off-peak period arrives the lower heater is turned on and the entire tank becomes heated. HEATING BY REVERSED CYCLE REFRIGERATION Reversed refrigeration is frequently referred to as a heat pump since the electric motor driving the refrigerating compressor furnishes the motive power-to transfer heat from one temperature to a higher temperature level. The compressor acts as a reversible refrigerating unit to extract :heat from the outdoor air in winter and deliver it indoors for heating purposes, and, by a reversal, to extract heat from the indoor air in summer and discharge it outdoors. In normal use a refrigerating machine is arranged to remove heat and the heat removed is dissipated to the condenser cooling water. The driving energy is converted into heat, most of which is added to the heat removed and extracted. In so-called reversed refrigeration the heat removed together with the heat converted from the driving energy is , utilized to heat the building. This conservation of the heat converted from the driving energy enables the reversed refrigeration to show a better performance in heating service than straight refrigeration can show, in cooling service. In order to overcome the drop in capacity and in efficiency with lower outside temperatures, it is often desirable to use well-water instead of air as the source of heat. For a detailed description of this cycle see Chapter 39. AUXILIARY ELECTRIC HEATING In conjunction with heating systems of other types, an auxiliary elec tric-heating arrangement is a convenient means of caring for mild days, in the spring and fall which require little heat to make a building com fortable. '. Likewise, such electric heating might be used on abnormally cold days to help out the main heating system and by this means reduce the necessary .size of the system. A few installations have been made using electric heating cable buried in the floors of bathrooms, etc., to provide auxiliary electric heating. At . least one airplane hangar is heated in this manner. Because of the feeling of comfort that a radiant type heater gives, bathrooms may be heated electrically with this type of heater while the rest of the house is cared for by some other system. Offices and rooms which require heat at periods when the main heating plant is shut down can be conveniently heated electrically. CONTROL Because the efficiency of electric heat production is the same for , small and large units, it is possible to reduce heat waste to a minimum' by applying local heating, locally controlled. Heaters are often controlled manually but thermostatic control is essential for economical operation. For duct systems having a variable volume of air flow the electric heater control must automatically vary the heat input in coordination with the changes in air volume and demand for heat.