Air-conditioners employ the same operating principles and basic components as a home refrigerator. An air-conditioner cools with a cold indoor coil called the evaporator. The condenser, a hot outdoor coil, releases the collected heat outside. The evaporator and condenser coils are serpentine tubing surrounded by aluminum fins. This tubing is usually made of copper. A pump, called the compressor moves a heat transfer fluid (or refrigerant) between the evaporator and the condenser. The pump forces the refrigerant through the circuit of tubing and fins in the coils. The liquid refrigerant evaporates in the indoor evaporator coil, pulling heat out of indoor air and thereby cooling the home. The hot refrigerant gas is pumped outdoors into the condenser where it reverts back to a liquid giving up its heat to the air flowing over the condenser's metal tubing and fins.

The basic types of air-conditioners are room air-conditioners, split-system central air-conditioners, and packaged central air-conditioners.

  • Room Air-conditioners: Room air-conditioners cool rooms rather than the entire home. If they provide cooling only where they're needed, room air-conditioners are less expensive to operate than central units, even though their efficiency is generally lower than that of central air-conditioners.

  • Central Air-conditioners: Central air-conditioners circulate cool air through a system of supply and return ducts. Supply ducts and registers (i.e., openings in the walls, floors, or ceilings covered by grills) carry cooled air from the air-conditioner to the home. This cooled air becomes warmer as it circulates through the home; then it flows back to the central air-conditioner through return ducts and registers. A central air-conditioner is either a split-system unit or a packaged unit.

  • In a split-system central air-conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air-conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air-conditioner's evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air-conditioner, a split-system is the most economical central air-conditioner to install.

  • In a packaged central air-conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house's foundation. This type of air-conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home's exterior wall or roof to connect with the packaged air-conditioner, which is usually located outdoors. Packaged air-conditioners often include electric heating coils or a natural gas furnace. This combination of air-conditioner and central heater eliminates the need for a separate furnace indoors.

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