We understand how home heating works: Use energy to heat air inside a furnace, then blow it into the house. But how do you make air cooler?
The answer has to do with the physics of evaporation. When a liquid turns into a gas through evaporation, it absorbs heat. This is one reason you sweat when your body gets too hot: As sweat (a liquid) evaporates on your skin, it takes heat with it, cooling your body.
An air conditioner relies on the same principles but the evaporation takes place within a closed loop as a chemical compound is converted from liquid to gas and back to liquid in a continuing cycle. Known as a "refrigerant," the chemical compound easily converts to gas at relatively low temperatures compared to water, for example, which must be extremely hot to convert to gas.
The mostly widely known refrigerant, Freon, is actually a trademarked refrigerant combination owned by DuPont. Used widely up until recently, many refrigerants including Freon relied on chlorofluorocarbons, or CFCS. In residential heating and cooling, CFC-containing refrigerants are also known as R-22 type refrigerants. However, since CFC products were found to contribute to the depletion of the ozone layer, they have been phased out of the HVAC industry.
Today, most new residential HVAC systems rely on a special class of chlorine-free, non-ozone depleting and more environmentally friendly refrigerants. Known in the industry as R-140 refrigerants, Puron, EcoFluor, Genetron are some common brand-names you may hear of.
What happens indoors:
If your furnace is in the basement or a utility closet, the central air components will be with it. The cooling process takes place within the main air handler unit, which houses the evaporator coil. The evaporator coil turns liquid refrigerant into its gaseous form, thereby creating evaporation which removes heat.
The central air handler blows air taken from the home's return supply duct over the evaporator coil, cooling it. The cooler air is then blown to the rest of the house, reducing the temperature.
What happens outside:
Once finished undergoing the evaporation process, the refrigerant (still in a gas state) is moved through the loop system to the outdoor air conditioning unit. Here, a compressor squeezes the volume of the gas to convert it back to a liquid, which allows it to cycle through the system again.
Just as evaporation removes heat, compression creates heat. That's why this part is outside. The heat is expelled from the outdoor system via a set of condensing coils. The big fan you see on top of the outdoor unit blows the excess heat out into the air.