Copyright Goodheart-Willcox Co., Inc. 800 Air-Conditioning Systems Some operating controls may function for both the cooling system and the heating system. The heating con- trols govern the operation of the heating system. A room thermostat calls for heat until the desired room tempera- ture is reached. Then, the electrical circuit is opened and the heating device deactivated. Details concerning the arrangement, installation, and operation of heating sys- tem controls are provided in later chapters. A thermostat controls comfort cooling. Central air- conditioning systems generally use a single thermostat to control the cooling system. The thermostat turns on the comfort cooling system when the temperature of the room rises above a certain set point. It shuts off the cooling sys- tem when the temperature has been lowered to the desired level. A simplified electrical wiring diagram of a thermo- stat circuit is shown in Figure 32-4. Many residential comfort cooling systems use a low- voltage thermostat. The thermostat controls a contactor that closes the compressor’s motor circuit. Pressure safety devices are also installed in the circuit. A high-pressure safety switch is wired in series with the starter coil. It will open the circuit if pressures become too high, Figure 32-5. Automatic controls make it possible for a central sys- tem to change from heating to cooling. If the outside air is at a certain temperature above the heating thermostat setting or a certain temperature below the cooling ther- mostat setting, some systems will draw in outside air to maintain a comfortable temperature inside the building. In such cases, the air-conditioning system is performing only as an air distribution and air-cleaning system. This permits the greatest economy of operation, as comfort is maintained and the minimum amount of fuel is consumed. 32.3 Central Air-Conditioning Systems A central air-conditioning system includes a single evap- orator installed in a central air handler cabinet that is con- nected to a network of ducts. Cooled and dehumidified air is distributed to the conditioned spaces through a series of supply ducts and returned to the air handler and evapo- rator through return ducts. Central air-conditioning sys- tems usually provide heating, cooling, humidity control, ventilation, and air cleaning to multiple spaces. Often, the heating system shares the air handler, blower, and duct system with a central air-conditioning system. A central air-conditioning system shown in Figure 32-6 Room thermostat Relay Relay coil Relay contact Line Compressor motor 1 hp or less Low-voltage transformer 24 V 120 V Overload heater Goodheart-Willcox Publisher Figure 32-4. A simplified wiring diagram of a thermostat relay circuit for small cooling systems. The room thermostat closes when the temperature rises to the cut-in temperature for cooling. The low-voltage relay coil is energized, which closes the relay contact in the high-voltage circuit, energizing the compressor. A relay would be used for small compressors only. For larger compressors that require greater ampacity, a contactor would be used in place of the relay. R 240 Vac 24 Vac Contactor contacts Thermostat OL High pressure Low pressure Contactor coil Contactor contacts Starting capacitor S C Compressor Goodheart-Willcox Publisher Figure 32-5. Simplified wiring diagram for a compressor control circuit. When the thermostat closes to call for cooling, the contactor coil is energized. The energized contactor coil closes the contacts and energizes the compressor. The low- voltage circuit includes three normally closed safety switches: a thermal overload monitoring the compressor temperature, a high-pressure switch monitoring the discharge pressure, and low-pressure switch monitoring the suction pressure.