Copyright Goodheart-Willcox Co., Inc. 802 Air-Conditioning Systems flow into its evaporator. The type of metering device used depends on system size and application. See Chapter 21, Metering Devices, for more information. A single combination heating and cooling thermostat is often used in central air-conditioning systems. Condi- tioned air is forced throughout a building by a blower. The blower is typically positioned just after an air filter and the return air plenum. With this arrangement, the air filter in the return air plenum cleans the air before it reaches the blower. An arrangement is sometimes provided to bring in fresh outside air as needed. The fresh air inlet may be controlled thermostatically, manually, or by air pressure. Some systems use a humidistat to control relative humidity in the conditioned space. Such systems usually have a humidifier in the plenum chamber that adds mois- ture to the air when the furnace is operating. The outdoor unit, or condensing unit, contains the compressor, condenser, and condenser fan. The condenser coil of outdoor, air-cooled condensers normally line the inner walls of the unit. A condenser fan draws air into the condensing unit, around the condenser coil, and then dis- charges upward and outward, Figure 32-8. Outdoor units for heat pumps are similar to those for air-conditioning, with some exceptions. See Figure 32-9. A heat pump outdoor unit contains a reversing valve, which determines if the compressor discharge line is directed to the outdoor coil (cooling) or indoor coil (heating). Heat pump outdoor units also include an accumulator, which stores excess refrigerant when the system is operating in heating mode. Heat pumps may also be equipped with a defrost mode, which prevents ice from building up on the outdoor coil. In heat pump–based central air conditioning, an electronic thermostat like the one shown in Figure 32-10 monitors both indoor and outdoor temperatures. The LCD on the thermostat face shows the indoor temperature and outdoor temperature. It also shows whether the unit is in a heating or cooling mode. Compressor Condenser Condenser fan Suction line Liquid line Condensate drain Thermostat (in conditioned space) Return air COOLING MODE Evaporator Goodheart-Willcox Publisher Figure 32-7. A central air-conditioning system in cooling mode. An A-coil evaporator coil in the plenum cools air blown over it by the blower. The outside condensing unit rejects the heat that was absorbed in the evaporator. Humidity is removed by condensing excess moisture on the evaporator surface. A drain tube carries away the condensed moisture.