air conditioning

How Air Conditioning Works, When You Need It and When You Don’t


If you’re reading this post in America you’ll be experiencing a lot of air conditioning this summer.  87 percent of U.S. households are equipped with AC and its a rare work place that doesn’t feature central air.  We all enjoy basking in the coolness on a scorching summer day, we may not give Heating, Ventilation, and Air Conditioning (HVAC) much thought most of the time, especially if we live and work in thermostat controlled buildings.


What’s the deal with Air Conditioning anyway?  How do those noisy metal boxes actually turn hot air into cold?  The diagram above –  adapted from Mechanical and Electrical Equipment for Buildings‘ invaluable section HVAC for smaller buildings (fig 9.5 in the 10th edition) – to get a sense of the way modern air conditioning works.

A coolant (formerly freon now a non-hydrochlorofluorocarbon equivalent) is a substance which can convert from liquid to gas forms at relatively low air temperatures.  Just as we discussed Tuesday in our post on Cooling WITHOUT AC  the process of changing state from liquid to gas absorbs heat, thus cooling the surrounding air. 

In an AC unit, that coolant passes around a loop in which it is compressed once a cycle from gas down to liquid and then allowed to convert back to gas on the other side of the loop.  On the compressing side, the motor generates heat and the coolant in the coils on that side is also warmer but outside air is vented in and past them, taking the heat away.   On the cool side of the unit, the coolant converts to gas and chills the surrounding air (air that has been gathered from a room or house and is then vented back inside.  

Voila, cool air.  

However the process of creating that cool air actually generates quite a bit of heat (and some noise) as you’ll know if you ever stand on the outside of an AC unit or feel the back of your refrigerator which operates the same way!

Note: Energy Star reminds us that bigger is not better in the world of Air Conditioning.  An AC unit  both cools and dehumidifies the space.  One that is oversized will rapidly cool the space before it is dehumidified, leaving the room with “a damp, clammy feeling.”  Use the website to properly size an AC unit for your square footage.  


Air Conditioning exploded on to the American building market after World War II.  Before that everyone used passive cooling methods – fans, breezes, siestas and sweat – to deal with the heat.  Hotter regions of the US were less populated for the simple reason that it wasn’t very pleasant to live there.  Buildings were designed with natural ventilation in mind – one of the reasons that the Courtyard Apartment building is so pervasive in Chicago  is that they allow each unit ample natural light and cross ventilation.  They were almost entirely built before the AC boom.

Then, in “a period of great technological optimism, low energy costs, and streamlined construction practices,” we rapidly removed many of the standard passive design features of American living spaces.   We replaced cooling high ceilings with an 8′ standard, reduced the number and size of windows formerly needed for both light and cross vent and didn’t even notice as our “dependence on the consumption of electrical power” skyrocketed.  As Christine Hunter describes it in Ranches, Rowhouses & Railroad Flats (page 83), we rapidly went from using mechanical systems to augment our home comfort to relying on them … and then taking them for granted.

Lisa Heschong puts it succinctly in her beautiful monograph, Thermal Delight in Architecture:

“There is an underlying assumption that the best thermal environment never needs to be noticed and that once an objectively ‘comfortable’ thermal environment has been provided, all of our thermal needs have been met.  The use of all our extremely sophisticated environmental control systems is directed to this one end – to produce standard comfort zone conditions.”

But, as she goes on to point out – that standardized comfort is no more nurturing to the human soul or enlightening to our experience than is a perfectly calibrated IV nutrition drip.  Never to feel the heat of the sun, or the cool of a breeze would be a terrible fate.  Why then, do we flee to conditioned spaces every time the thermometer wobbles?  We don’t need to live entirely at the mercy of the elements but perhaps there’s no harm in adapting to the daily conditions … and experiencing them … a little more deeply, summer or winter.


Via Popular Mechanics’ fantastic Brief History of Air Conditioning.  Follow the link to see their whole (comprehensive and comprehensively snarky) list of key dates in the timeline of AC development .

1758 All liquid evaporation has a cooling effect. Benjamin “I invented everything” Franklin and Cambridge University professor John Hadley discover that evaporation of alcohol and other volatile liquids, which evaporate faster than water, can cool down an object enough to freeze water. 

1881 After an assassin shoots President James Garfield on July 2, naval engineers build a boxy makeshift cooling unit to keep him cool and comfortable. The device is filled with water-soaked cloth and a fan blows hot air overhead and keeps cool air closer to the ground. The good news: This device can lower room temperature by up to 20 F. The bad news: It uses a half-million pounds of ice in two months… and President Garfield still dies. 

1914 Air conditioning comes home for the first time. The unit in the Minneapolis mansion of Charles Gates is approximately 7 feet high, 6 feet wide, 20 feet long and possibly never used because no one ever lived in the house. 

1931 H.H. Schultz and J.Q. Sherman invent an individual room air conditioner that sits on a window ledge—a design that’s been ubiquitous in apartment buildings ever since. The units are available for purchase a year later and are only enjoyed by the people least likely to work up a sweat—the wealthy. (The large cooling systems cost between $10,000 and $50,000. That’s equivalent to $120,000 to $600,000 today.) 

1942 The United States builds its first “summer peaking” power plant made to handle the growing electrical load of air conditioning.

1950s In the post-World War II economic boom, residential air conditioning becomes just another way to keep up with the Joneses. More than 1 million units are sold in 1953 alone. 


And what about the present.  Apparently Americans haven’t yet reached total market saturation for in-home air conditioning.  Even in the last ten years the number of homes with air conditioning has risen in every region.

More power to us, is one way to look at it … or perhaps it is more power FOR us.    Check out the US Energy Information Administration’s Residential Consumption Survey for more data on US energy use.

Air Conditioned Homes in the US

What about you?  Do you have AC in your home?  Is it central or a window unit?  Do you use it regularly?  Tell us in the comments!