| Central  air conditioners have two separate components: the condenser and the  evaporator. The condenser unit is usually located outside the house on a  concrete slab. The evaporator coil is mounted in the plenum or main duct  junction above the furnace. 
 Most  central air conditioners are connected to a home's forced-air distribution  system. Thus, the same motor, blower, and ductwork used for heating are used to  distribute cool air from the air conditioning system. When a central air  conditioner is operating, hot air inside the house flows to the furnace through  the return-air duct. The hot air is moved by the blower across the cooled  evaporator coil in the plenum and is then delivered through ducts to cool the  house. When the air conditioner works but the house doesn't cool, the problem  is probably in the distribution system.
 
 Caution: Before doing any  work on an air conditioning system, make sure the power to the system, both to  the condenser and to the evaporator assembly, is turned off.
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