How does Central AC Work

central ac works like thisDesigned for cooling the entire house, central air conditioners are located outdoors. They consist of a large refrigerant compressor and condensing coils unit joined by copper tubing refrigerant lines to an indoor coil mounted in the furnace. The same ducts are used for distribution of both heated and cooled air. This is known as a “split system”. In a “packaged system”, all the components are located outside the house in a single unit. In both types, an additional component called a thermostat is used to control the level of cooling in continuous feedback loop.

A central AC works by using energy to subtract heat away from the air. The most widespread type uses a compressor cycle, like a refrigerator does, in order to transfer heat from the house to the outdoors. Employing specialized fluid called refrigerant, heat is absorbed and then released when the refrigerant transforms back and forth between a gas and liquid state. As it changes to a liquid from a gas, it releases heat; when changing back from liquid to gas, it absorbs heat.


A refrigerant compressor is a mechanism which pumps the liquid refrigerant through an expansion valve, altering the liquid to a mixture of low-pressure liquid and gas. In the indoor evaporator coil located in the furnace, the rest of the liquid soaks up heat from interior air like a sponge, in the process becoming a low-temperature gas.

This low-temperature gas is then compressed by a compressor which reduces its volume, increases its temperature and causes it to become a high-temperature, high-pressure vapor. The resulting vapor is driven to the outdoor coil or condenser, and its heat is therefore transmitted to the outdoor air. The outdoor air temperature essentially makes the refrigerant condense into a liquid. The liquid goes back to the expansion phase compressor and the cycle begins all over.

How does this work? Interior air is dehumidified and cooled as it passes over the indoor coil. Humidity removed from the air, as it makes contact with the indoor coil, is collected in a pan at the bottom of the coil and sent to a drainage line.

Pros and Cons

Central air conditioning has numerous advantages over many smaller disbursed units:

• When the air handling unit turns on, the interior air is drawn in from various parts of the house through return-air ducts. This air is pulled through a filter where airborne particles, like dust and lint are filtered out. Some better filters may remove microscopic pollutants as well. The filtered air is sent to air supply ducting and back to interior rooms. When the ac is running, the process repeats continually.

• Because the central ac's heat exchange fan and compressor is located outside the home, it usually offers a lower level of noise indoors than window or through-the-wall air conditioning units. In such units the heat exchange fluid (refrigerant) is piped under pressure to an indoor heat exchanger through which building air is circulated. This indoor heat exchanger is typically fitted with a condensate collection device and disposal system, as in cooling mode a significant amount of water can condense from the indoor air. In heating mode condensate is collected by the outdoor heat exchanger, which is typically monitored for icing up, and is usually fitted with an evaporation pan.

One possible disadvantage to central ac is that the air ductwork does become dirty over time, and poses a risk of growth and spread of harmful microorganisms, mold and mildew. Early models of air conditioners used freon as the regrigerant fluid, however, because of the harmful effects freon has on our environment, it has been phased out.

photo by hokkey / CreativeCommons