[This reflects a few corrections from the original posting]My wife and I did our outside work early, while the weather was still bearable. Since mid-day we’ve been holed up in the house. It’s not exactly cool indoors, but we’ve had the house closed up and it’s about 15 degrees cooler than outdoors. If it gets much warmer, though, I admit that I’ll at least be thinking about getting an air conditioner—as I do every year for a few days during the hottest weather.Readers of last week’s blog know that there are usually ways to avoid mechanical air conditioning (at least in Vermont)—though in places like Phoenix (where it was forecast to reach 109°F today), air conditioners are pretty-much a necessity.So what is an air conditioner exactly, and how does it work?A short history of refrigerant-cycle air conditionersRemarkably, one of our founding fathers, Ben Franklin, had a hand in the underlying science of air conditioning. In 1758, Ben Franklin and a colleague in England, chemist John Hadley, conducted an experiment on the cooling properties of evaporation. By using a bellows to evaporate highly volatile liquids like alcohol and ether, they were able to drop the temperature to 7°F, building up a thick layer of ice on their mercury thermometer—while the ambient temperature was 64°F.In 1820, another of history’s greatest scientists, the British inventor Michael Faraday, showed that by mechanically compressing ammonia to liquefy (condense) it and then allowing the ammonia to expand and evaporate, he could cool air. And in 1842, a Florida physician, John Gorrie, wanting to keep patients cool, was able to use this principal to make ice in an Apalachicola hospital. Gorrie patented his system in 1851 and hoped to commercialize it to cool buildings, but his financial backer died, and with it, Gorrie’s path to success. Air conditioning would not reappear for 50 years.In 1902, Willis Carrier of Syracuse, New York perfected a system for dehumidifying a commercial printing plant. The goal was to stabilize the paper, but the invention also kept the plant’s temperature more comfortable and the workers more productive. He formed The Carrier Air Conditioning Company of America to produce these systems, eventually extending beyond commercial buildings to homes. With 32,000 employees in 170 countries, Carrier Corporation (now a subsidiary of United Technologies Corporation) is today one of the world leaders in high-technology heating, air-conditioning, and refrigeration systems.Refrigerant-cycle air conditionersThe operation of a mechanical air conditioner depends on a refrigerant, a specialized fluid whose phase can be altered between liquid and vapor by compressing and then evaporating it in a closed loop. This phase change process absorbs and releases heat, thus enabling an air conditioner to move heat from one place to another—even if that heat flow is from a cooler to a warmer space (seemingly in opposition to Second Law of Thermodynamics). This vapor-compression or refrigerant cycle is the principle behind nearly all air conditioners, as well as heat pumps and your kitchen refrigerator.It’s a complicated process, though, so hang onto your hat!In an air conditioner, refrigerant is pumped through a closed loop where it is alternately evaporated and condensed. In the process of evaporation, which usually occurs inside the house, heat from the house is absorbed into the refrigerant—cooling the house. That gaseous refrigerant is then pumped to the condenser, located outside the house, where the compressor mechanically compresses the vapor, increasing its pressure and causing it to condense back into liquid; this process releases the heat that was absorbed inside the house.The evaporator coils and condenser coils are heat exchangers that transfer heat first from the indoor air into the refrigerant and then from the refrigerant to the outside air. It’s an elegant process that is largely hidden inside the air conditioner components. In a window air conditioner, this refrigerant cycle occurs in a single box, absorbing heat on the room side and rejecting it on the side hanging outside the window. In a central air conditioner the indoor and outdoor units are separate and connected by refrigerant lines.To summarize, this refrigerant cycle is a closed loop, and two phase changes occur every time the refrigerant is pumped around that loop. Heat is absorbed on one side of the loop (evaporation) and rejected on the other (condensation). In a heat pump this refrigerant cycle can be reversed allowing heat from the outside air the be captured and delivered into the house during the winter months. (I told you this was going to be complicated!)Very significantly, air conditioners lower household humidity levels. This occurs because the evaporator coils that are in contact with the house air are cold (below the dew point temperature), and moisture from the indoor air condenses on them (just as water condenses on the outside of your glass of iced tea on a hot humid day). This condensate is captured and drained outside the building, thus removing that moisture from the house and lowering the relative humidity. That condensate is what drips on you when you walk underneath a window air conditioner in the summer.Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also coauthored BuildingGreen’s special report on windows that just came out. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.