Do revolving doors really save energy compared with swinging doors?

Illuminating answers to environmental questions.
July 29 2008 11:51 AM

Are Revolving Doors More Energy Efficient?

What about the ones that turn automatically?

My office building just had its entryway refurbished. Now, we have a huge revolving door flanked by two regular, swinging doors. I heard somewhere that revolving doors are supposed to make heating and air conditioning more efficient. Is that true, or can I use the regular doors guilt-free?

Revolving doors
Do revolving doors save energy?

Right now, it's 71 degrees Fahrenheit in the Lantern's Manhattan office and 87 degrees outside. It takes a lot of electricity to cool this entire office building by 16 degrees—but it would take even more if people didn't use the two large revolving doors between the building lobby and the hot sidewalk outside.

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Generally speaking, air flows in and out of a building because of differences in air pressure. (In the winter, heated air rises toward the top of a building, and—as long as there are any openings on the ground floor—cold air rushes in to replace it. The opposite happens in the summer, with the cold air flowing out the front doors.) Regular foot traffic in a large office building can result in air leaks of up to 30,000 cubic feet per minute.

That presents a challenge for engineers: How do you allow people and things to move in and out of a building while minimizing unwanted air flow? The revolving door was presented as a solution for this age-old problem more than 100 years ago, long before anyone was talking about carbon footprints or global warming. (The primary purpose, according to the original patent application—filed by one Theophilus Van Kannel of Philadelphia—was to prevent the "entrance of wind, snow, rain or dust.") A revolving door isn't airtight (PDF), but the barrier it creates makes the impact of that air pressure differential less important. Every time the door spins, some air will leave the building and some will come in, but overall, much less passes.

How big a difference can using a revolving door make? In 2006, a team of graduate students at MIT conducted an analysis of door use in one building on campus, E25, where they found just 23 percent of visitors used the revolving doors. According to their calculations, the swinging door allowed as much as eight times more air to pass through the building than the revolving door. Applying average Boston weather to their equations, the MIT team found that if everyone used the revolving doors, it would save more than 75,000 kilowatt-hours of energy—about 1.5 percent of the total required to heat and cool the building—and prevent 14.6 tons of carbon dioxide from being emitted. * (By way of comparison, the EPA says an average American vehicle emits about six tons of carbon dioxide over a year.) The gains are also big enough that they could easily cover the energy needed to power an automatic revolving door like this one, which has a 250-watt motor.

So, how do you get people to use revolving doors? The MIT group didn't come up with an obvious answer. They were able to increase revolving-door usage by putting up signs, but the rates at E25 never rose above 63 percent (PDF). (A sign that politely asked people to use the door turned out to be more effective than one detailing the energy savings.)

Here's the good news: The research team found that in another building on campus—one where a simple and polite sign had long been posted before researchers started tracking door usage—revolving-door use was higher than anywhere else on campus, even after those signs were taken down. And the MIT team also noticed that there appeared to be a snowball effect—once one person used a revolving door, other people often followed, particularly since it required less force to push through. That's a particularly good finding given that the gains from using the revolving door aren't linear: You save much more energy by raising usage from 50 percent to 75 percent than you would by raising it from 25 percent to 50 percent.

Of course, buildings need to provide options besides revolving doors for people in wheelchairs, parents with strollers, or anyone carrying an unwieldy load. But if you can help it, it makes sense to forget holding the door open for someone and take them for a spin instead.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

* Correction, July 29, 2008: The original story incorrectly said that using the revolving door would save about 74 percent of the energy needed to heat and cool Building E25 on MIT's campus. Using the revolving door would save about 1.5 percent of the total energy required to heat and cool the entire building, and about 74 percent of the total energy required to heat and cool the air exchanged when people pass in and out of the building. (Return to the corrected sentence.)

Jacob Leibenluft is a writer from Washington, D.C.

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