The refrigerant liquid passes through an expansion device into the evaporator coil and expands to a gas. In the process, it absorbs heat from the air forced over and through the evaporator coil. The gaseous refrigerant circulates from the evaporator coil to the compressor where it is compressed back under pressure into a high-pressure gas. The gas then passes into the condenser coil where it usually transfers heat to the outdoors and becomes a liquid. A condenser fan forces air across the condenser coil where it picks up heat.
Some condensers use water to cool the refrigerant rather than air.
It can be said that an air conditioning unit or system operates by removing heat from the air inside and releasing it outdoors. Heat pumps, which cool and heat, operate in much the same manner but are reversible for winter use. When adding central air to an existing house, be certain that you have the proper size fan, ductwork and returns since cold air, being heavier than hot air, dictates system adjustment for efficient operation. Central air conditioning systems do not require a burdensome amount of maintenance, but some simple care must be done periodically if the unit's maximum economic life span is to be achieved. Normally a central system can be expected to last 5 to 8 years.