Copyright Goodheart-Willcox Co., Inc. 804 Air-Conditioning Systems The indoor unit of a central air-conditioning system is also called the air handler. Figure 32-11 shows an air handler with gas furnace used to distribute conditioned air throughout a building. To add cooling to this furnace, an evaporator would be installed in the plenum above the heat exchanger, Figure 32-12. The air handler “pulls” air in from the return duct system and “pushes” air out through the supply duct system. 32.4 Installing Central Air Conditioning Before installing an evaporator and condensing unit to an existing furnace or air handler, examine the existing system. If a furnace is old and worn or its efficiency rating is low, consider replacing the furnace with a higher effi- ciency model. After deciding, systems are assembled on- site in four steps: 1. Install the condensing unit. 2. Install the evaporator. 3. Install the suction and liquid lines. 4. Install the electrical wiring. Replacing an HVAC System When replacing an HVAC system, always calculate the heating and cooling loads to properly size the replacement system. Do not assume that the replacement system should have the same capacity as the system it is replacing. The original system may have been sized incorrectly. Changes made to the building since the previous system was installed, such as the addition or removal of windows, doors, and insulation, may make a larger or smaller system more efficient. Even changes in shading due to maturing trees or new construction around the building can have a significant effect on the heating and cooling loads. Thinking Green Tempstar Figure 32-11. This air handler contains a gas furnace and heat exchanger for warming air circulated by the blower. Suction line manifold Distributor Sensing bulb Suction line connection (capped) Liquid line connection (capped) TXV Rheem Manufacturing Company Figure 32-12. This evaporator is already fitted into a plenum and ready to be added onto a forced-air furnace installation. Rheem Manufacturing Company Figure 32-10. A typical electronic wall thermostat with an LCD screen that displays indoor temperature, outdoor temperature, and heating or cooling mode. The thermostat can be used to raise the temperature, lower the temperature, take indoor and outdoor temperature readings, select heating or cooling mode, and select continuous or automatic fan operation. Heat exchanger Conditioned air Furnace exhaust Gas valve Blower Electronic controls Gas burners