DEVICES USED FOR HEATING 



135 



high cost of electricity in most communities prohibits 

 their general use. 



Exercise. From your study of home heating reach 

 conclusions bearing on the following questions. The 

 answers to these should be recorded. Which of the three 

 commonly used methods of house heating is best adapted 

 for providing the proper amount of moisture or humidity 

 in the air? Can you suggest a way in which air could be 

 humidified for each of the types of heating system? Much 

 of the heat in the home is lost through the walls and 

 ceilings of the rooms to the outside. Modern methods 

 employ insulation to reduce this loss. Of the materials 

 listed in the table on page 131, which ones do you think 

 might be used for house insulation? Can you suggest 

 other materials that are used for this purpose? 



What is air conditioning? Today we are hearing so 

 much about air conditioning that it is important for 

 everyone to learn the facts concerning it in order 

 that we may know how to judge intelligently about 

 its problems. Air conditioning means the controlling 

 of the factors which make for the greatest health and 

 comfort in the air of our homes and public buildings. 

 These factors are the purity, relative humidity, and 

 temperature of the air. 



Courtesy General Electric Company 



FIG. 213. CONDENSING UNITS FOR AIR CONDITIONING 

 IN A THEATER 



In the past the greatest concern about air condi- 

 tioning has been to heat the air in our homes and pub- 

 lic buildings in winter. This we now know is not 

 sufficient. Air temperatures may be controlled in sum- 

 mer. Clean air, properly humidified, may be provided 

 at all times of the year. In recent years many industri- 

 al plants and public buildings have been air-condi- 

 tioned, and now many companies offer air-condition- 

 ing equipment for homes. 



In the conditioning of air for a large building such 

 as a theater or industrial plant, the air is drawn in 



Minneapolis-Honeywell 



FIG. 214. THERMOSTAT 



from the outside by large 

 fans. It is sometimes washed 

 through a spray of water to 

 remove dust and other unde- 

 sirable material. After clean- 

 ing, the air is either heated or 

 cooled, provided with the 

 proper amount of moisture, 

 and blown into air ducts 

 which carry it to various parts 

 of the building. 



In summer the air is cooled 

 by passing it over refrigera- 

 tion coils. You will recall from 

 your study of Topic 1, Unit 

 II, how temperature may be 

 lowered in a refrigeration 

 system by allowing a com- 

 pressed gas such as ammonia 

 to evaporate. Such a refriger- 

 ation plant is usually used in 

 theaters, industrial plants, and 



office buildings to cool the air in summer. Figure 213 

 shows the condensing units of the air-conditioning 

 equipment used in a large theater. In the winter the 

 air is heated by blowing it over heating coils. In both 

 summer and winter the air is provided with moisture 

 to give the proper relative humidity. 



Today many of our homes are equipped with heat- 

 ing plants which are controlled automatically. The 

 drafts of coke or coal-burning furnaces are opened 

 when the temperature drops and closed when it rises. 

 Oil and gas burning furnaces are also controlled auto- 

 matically. The device which makes this possible is 

 known as a thermostat (Fig. 214). The thermostat 

 contains a coil which is very sensitive to changes of 

 temperature. It is made of two pieces of metal and is 

 sometimes known as a compound bar. One of the 

 metals expands and contracts with a given tempera- 

 ture change much more rapidly than the other. This 

 tends to make the coil straighten. The thermostat is so 

 constructed that, when the temperature of a room be- 

 gins to drop and the bar to contract, an electric con- 

 tact which turns on an electric motor is made. This 

 motor may either turn on the drafts of a coal or coke 

 furnace or start the burner of an oil or gas furnace. 

 When the temperature begins to rise, the compound 

 bar expands, and after the temperature has reached a 

 certain point, the bar makes an electrical contact on 

 the other side and the motor is shut off. Figures 215 

 and 216 show an oil burner and a gas burner, both of 

 which are controlled by thermostats. 



Air conditioning in homes at the present time does 

 not seem to be entirely practicable because there are 



