We pretty much just take the fact of central air conditioning for granted these days. It is present almost everywhere and it is hard to imagine getting through a long hot summer without it. But if you are like most people, you probably do not actually know how central air conditioning works. While you can certainly take advantage of it without understanding it, the basic concept is pretty simple.
Basically, central air conditioning systems are composed of an outdoor unit that typically houses the compressor and condenser and an indoor unit that manages the flow of air throughout your house or other building. This indoor unit is typically either an air handler or a furnace, and it directs the flow of air through a series of ducts that feed into the various rooms of the house.
The cool air originates in the outdoor unit and is blown into the house, gradually absorbing heat as it goes, and that air is then returned to the outdoor unit to be re-cooled. What actually happens in the outdoor unit involves the cycle of a type of refrigerant from a gas to a liquid and back. In the condenser area of the outdoor unit, the pressure on the refrigerant is lessened and it is able to absorb heat from the air returning from the house.
This gas, while warmer than the liquid refrigerant, is still quite cold and acts to cool the air being passed back into the house. As that refrigerant moves along to the compressor area, the gas is converted to a liquid and is forced to release the heat it had been holding. In that way, the air conditioner is able to remove heat from the inside of your house and release it outside.
Your air conditioning system is also generally hooked up to a thermostat, which is what controls when the unit switches on and off. You can set the thermostat at the temperature you would like to maintain inside your house and the thermostat will signal the air conditioner to switch on when the indoor temperature rises above that level. And once the indoor temperature is again below the preset level on the thermostat, the air conditioner will switch off again.