The facade of a building in Manhattan, with an A/C unit in every window.
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The U.S. government’s long-running Residential Energy Consumption Survey includes a lot of data on our A/C habits—and some surprises.

Summer has arrived, which in the United States means two things: air conditioning, and arguing about air conditioning.

The century-old technology for cooling our buildings has brought newfound summer comfort and enabled migration to and jobs in hot climates. It’s also the cause of constant bickering, since humans can’t agree on how much—if at all—we should be using air conditioning as the summer heat blares down. A/C is both a cause of and a response to a warming planet.

CityLab pulled data from the U.S. government’s long-running Residential Energy Consumption Survey to help shed some light on how Americans use air conditioning. The 2015 survey found that nearly 90 percent of American households have air conditioning of some sort.

This holds true even for older homes, built before air conditioning became common. Homes built before 1950 are only slightly more likely to lack A/C than homes built this decade. The key difference is those older homes are more likely to rely on window or wall air conditioning instead of central air.

“It’s pretty hard to buy a new home in the U.S. in the last 20 years that doesn’t have air conditioning, no matter where you live,” said Andrew deLaski, executive director of the Appliance Standards Awareness Project. Using his home state as an example, deLaski added: “It used to be that not too long ago, if you built a new home in Massachusetts, it wouldn’t have central air conditioning. That’s unheard of today.”

Given the cost of installing and running air conditioning, it’s not surprising that wealthier households are more likely to have it, and more likely to have central air instead of window or wall units. But even among low-income households, more than 75 percent have some form of A/C.

One of the biggest points of disagreement about air conditioning is not whether to have it, but how to set it. Some Americans like it cold and some like it hot, and the difference often plays out along gender lines, with men comfortable at cooler temperatures than women.

The debate is fiercest in office buildings where a single temperature is set for everyone, but it plays out in American homes, too. Around 20 percent of American homes are set to 69 degrees Fahrenheit or lower when people are home in the summer. Another 18 percent are 77 degrees or warmer. There are partisans for 72 degrees, and 75, and everywhere else—a balancing act between comfort and cost.

Gender isn’t the only factor affecting thermostat settings. Americans in colder climates are likelier to keep their homes chilly, while residents of hotter areas tend to prefer the mid-70s. It’s unclear whether how much that reflects economics—it’s cheaper to cool a home to 68 degrees when it’s 80 degrees outside than when it’s 95—and how much it reflects residents of warm climates possibly preferring warmer indoor temperatures.

Heating, not A/C, is the big energy hog

The United States uses more than four times as much energy heating homes than it does cooling them, although that gap has been closing.

That shouldn’t be a surprise, given the U.S.’s position in the Northern Hemisphere, with many cities that get quite cold in the winter. Heating a home from 30 to 70 degrees requires changing the interior temperature by 40 degrees, twice as much as cooling it from 90 to 70.

“The difference in temperature that you have to overcome is larger, so the work that’s required to warm a home in northern climates is more work than is required to keep a home comfortable in the summertime,” deLaski said.

But even warm-weather cities often spend considerably more energy on heating than cooling. Of the five climate zones tracked by the U.S. Energy Information Administration (EIA), air conditioning overtakes heating only in the “hot-humid” zone, which includes about 20 percent of American households.

Partly, this reflects the fact that many residents of cooler climates go without air conditioning altogether. While only about 12 percent of American households overall lack A/C, that rises to 15 to 20 percent in colder or drier climates, and nears 50 percent for “marine” climates, such as are common on much of the West Coast.

However, the gap between energy spent on heating versus cooling has been slowly closing. American homes are spending more energy on air conditioning and less on heating than they used to.

This is due to a range of factors, some of which work in different directions, deLaski said. Warm states have been growing much faster than cold states in recent decades, shifting the population from areas with high heating demand to ones with higher A/C demand. Air conditioning has become more common over that period, and homes have become bigger, requiring more energy to cool them.

All of that is counterbalanced by the fact that furnaces and air conditioners have become much more efficient over that time—especially air conditioners. The net effect is that U.S. residential A/C energy use is going up, but not by as much as heating energy use is going down.

Technology has the potential to continue improving the efficiency of both heating and cooling. Variable-speed air conditioners, already on the market as a premium product, have the potential to reduce A/C energy usage by 25 to 30 percent, deLaski said. On the heating side, one trend is toward heat pumps—essentially an air conditioner running in reverse. Because those run on electricity instead of fossil fuels, they can significantly reduce emissions in areas where a big portion of the power grid comes from clean sources.

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