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Air-conditioning systems

There are basically two types of central air-conditioning systems used in residential structures-the integral system and the split system.

Integral system

This system, sometimes referred to as a single-package unit, is self-contained. That is, all the components-compressor, condenser, expansion device, and evaporator, plus the electrical controls and fans-are contained in a single housing. This system, which must be vibration-mounted, is often installed in the attic or a crawl space with ducts projecting through the exterior wall or roof to provide air for cooling the condenser. The integral system is less expensive than the split system. However, the noise level from the compressor makes an interior installation less desirable.

Split system

In this system, the compressor-condenser is physically apart from the evaporator coil. To eliminate the interior noise and provide outside air for cooling the condenser, the compressor and condenser are housed in a unit located outside the structure, usually at the rear or side. The evaporator coil, on the other hand, is located in the house, either in the attic or inside the heating system. The specific location often depends on the type of heating system. When the house is heated by forced warm air, the evaporator coil is usually located in the furnace plenum and utilizes the furnace blower to move the air through the coil. When the house is heated by steam or hot water, the evaporator coil is usually located in the attic with a separate blower (fan) to move the air through the coil and distribute it throughout the house. When there is no attic, the evaporator unit can be located in a closet or the basement.

The compressor-condenser is connected to the evaporator coil by two copper pipes that contain the refrigerant that cycles between the two units. (See FIG. 17-2.) The diameters of the two pipes are different-one pipe is the size of a pencil, the other the size of a broom handle. The small pipe (pencil size) is the liquid line; it carries the high-pressure liquid refrigerant from the condenser to the expansion valve. The larger pipe (broom-handle size) is the suction line; it carries low-pressure refrigerant gas from the evaporator coil to the compressor. The suction line should be insulated. Usually it is covered with black foam-rubber type of insulation.

Inspection procedure Whether the air-conditioning system can be checked operationally depends on the outside air temperature. Most manufacturers do not recommend turning on the system at temperatures below 60° F because of the possibility of damage to the compressor. If the outside temperature is below 60° F during your inspection, do not start up the air conditioner. If the system cannot be checked prior to purchasing the house, the seller should provide you with a guarantee of its operational integrity. If the temperature is above 60° F during your inspection, walk over to the compressor and have someone turn down the thermostat that controls the air conditioner so that the system will begin to operate.

Compressor The compressor is the most important part of any air-conditioning system and the most costly to replace. Its projected life is about eight to ten years, although units have been known to last over fifteen years. In areas of the sunbelt with long air-conditioning seasons, the projected life will be somewhat less. After the system has been turned on, listen for any unusual sounds. The compressor should start up smoothly. A straining, grunting, groaning, or squealing noise indicates a problem condition that should be checked and corrected by a competent service organization. Shut the unit off and note the condition on your worksheet. Once the compressor starts up smoothly, it should then operate continuously without any noise (except a low hum) or squeaks. The compressor should not operate in short cycles (on and off repeatedly). If this occurs, there is a problem condition and the unit should be shut down.

Fig. 17-2. Schematic diagram of a split air-conditioning system. The compressor-condenser is located outside the house, and the cooling coil is inside the house (usually in the furnace plenum or attic).

OutsideHigh-pressure refrigerant line Evaporator coils
air Fan
Condenser coils Cool air
  Fan
  Low-pressure Condensate
  Compressor refrigerant line collection pan
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