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Posted by Augustus Cai on June 17th, 2021

During this hot summer, some people are looking for any way to stay cool. One solution is as simple as flicking a switch: reversing the direction of a ceiling fan. This change can keep a room’s occupants more comfortable, all while saving energy.

Many people don’t know you can flip a fan’s direction, but “it’s almost an inherent feature of most fans,” said Brent Haley, an electric sales associate at The Home Depot.

Ceiling fans usually have a switch on the motor or a button on a remote that changes the rotation direction. For the most effective cooling, the blades should spin counterclockwise in the summer when viewed from below.

How fans cool your space
Ceiling fans work by circulating air.

“What we actually perceive as humans is not so much the temperature,” said Todd Otanicar, a professor at Boise State University who studies the transportation of heat, “… but heat transfer — basically, are we being heated or cooled.”

As Otanicar explains, this is the difference between a wood stick and a metal hanger in a cold room. Both are the same temperature, but when someone picks them up, the metal rod feels cooler because the rate heat leaves the hand for the metal is faster than for the wood.

Fans increase the rate of heat transfer. Heat moves from a room’s occupants to the air faster when the air is flowing than when it’s stagnant. This cools the inhabitants.

If a person is sweating, the person will get even colder because of the heat required to turn the liquid sweat into water vapor.

In a place like Idaho, this cooling process “is more effective than, say, in the South, because we have such low humidity ranges,” explained Damon Woods, a professor at the University of Idaho and an expert on thermal comfort.

While a fan spinning in either direction can increase air flow, counterclockwise-turning fans push air down toward the room occupants because of the tilt of the fan blades. When the air flow is strongest as it hits people, they feel most comfortable.

Of course, this only works if people can feel the breeze from the fan.

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“If you’re sitting far away from the fan, and you can feel it in the middle of the room, but you can’t at your desk in the corner, then it’s probably not doing much good,” Woods said. “It would be better to have a little desk fan or something like that.”

Winter fan rotation should be reversed
In the winter, however, a little air flow can keep room occupants warm — as long as they don’t feel a breeze. To prevent a draft, a ceiling fan should be run on low and clockwise, so that air is pulled up to the ceiling and then pushed against the walls.

“As humans, we’re more wired to sense surface temperatures than air temperatures,” Woods said. This is why it’s uncomfortable to sit near a cold window, “even when the thermostat itself may not pick up on it.”

With warm air pressed against a room’s walls, the surfaces are warmer than they would be otherwise. This makes the area feel cozier.

Energy savings are difficult to quantify
There’s another reason to switch the spin in the summer: It saves energy and reduces electricity bills.

The draft created by a ceiling fan, according to the U.S. Department of Energy, “will allow you to raise the thermostat about 4°F with no reduction in comfort.”

The exact amount of energy saved from a switch is difficult to calculate. Haley said the rotation change could deliver a 40% to 50% energy reduction.

Woods found this estimate high but not unreasonable: “Maybe you would reach that 40% savings if you just use the ceiling fans and kept your air conditioning off.”

Energy savings depend on the room, fan and layout of the home.

Sophia Charan writes for the Idaho Statesman on a fellowship through the American Association for the Advancement of Science. She has a Ph.D. in chemical engineering from the California Institute of Technology, where she focused on atmospheric chemistry, and a bachelor’s degree from Yale University.

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Augustus Cai

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Augustus Cai
Joined: June 17th, 2021
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