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The Deal

• $49 for a basic AC system tune-up ($100 value)

Central Air Conditioning: A Quick-Change Act at Home

Take a peek at the principles of central air conditioning to understand just what your technician will be adjusting.

A refrigerant is a substance that transforms more easily than most from a gas to a liquid and back again. This shape-shifting is what makes modern air conditioning possible. Refrigerant cools the home by traveling through a compressor, a condenser, and an evaporator—the last a part of the furnace system inside the house, while the other parts reside in an air-conditioning unit that usually sits outside staring longingly at the family it works for.

It might seem natural to think of air conditioning as a process of blowing cool air into the home, but it might be more easily understood as a matter of carrying off hot air. Fans suck air from the home into the system and draw it across the coils of the evaporator, which house liquid refrigerant. As the refrigerant relieves it of its heat, the furnace’s blower and ducts distribute the now-cooler air throughout the home. Meanwhile, under heat, the refrigerant becomes a vapor that flows into the compressor, which further pressurizes the gas and propels it into the condenser.

Now it’s time to get rid of all that heat. In the condenser, heat is radiated away, helped along by the venting and large surface area of the metal fins on the outside of the unit. This lets the refrigerant cool down and return to liquid form, leaving it ready to flow back inside and pick up another load of the home’s heat.

Since it hit the market in 1932, central air conditioning has not only made it possible to build houses in the hottest regions of the country—it’s actually changed the way those houses are built. High ceilings, eaves, awnings, attics, and front porches were once standard elements of home architecture that provided relief from the stifling summer heat and invited bats to come flap their wings to make breezes. Central air made it both possible and, given the cost of installation, desirable to create simpler one-story homes without these architectural flourishes, leading to a post–World War II boom in sprawling modern developments.