Frederick Rohles is sitting in Manhattan, Kan., where the temperature hit 91, and a marvel of technology is cooling him overhead.

It's a ceiling fan.

In 1982, Rohles, then an engineering professor at Kansas State University, did the groundwork quantifying the cooling effect of a ceiling fan. It holds true today and is perhaps more relevant than ever.

Once dismissed as relics of the Victorian era, ceiling fans gained new cachet after the Arab oil embargo and the 1979 energy crisis.

Sales in the United States went from 250,000 in 1975 to 7.4 million in 1981.

But once air-conditioning took hold - 82 percent of U.S. homes now have it - "the trend was to lock it up, close the windows," says Stephen Melman of the National Association of Home Builders.

Now, however, people want to open their windows again. So the ceiling fan's popularity is coming around once more.

For the last two years, the showcase homes built for the annual meeting of the builders association prominently featured ceiling fans.

The recession also is fanning their popularity. Utilities have been encouraging customers to use ceiling fans to help reduce their electric bills, said Tom Reddoch of the nonprofit Electric Power Research Institute.

When Sean Crane of Media does energy audits for homes, one of the first cooling suggestions he offers is to get a ceiling fan.

So let's revisit Rohle's research.

The experiment involved 256 men and women who sat in a temperature- and humidity-controlled chamber and were exposed to different temperatures, with and without a ceiling fan.

They recorded their sensations on several different scales. Rohles concluded that a fan - depending on speed - can make people feel comfortable in temperatures as much as six degrees warmer than without the fan.

So if you're fine at 76, you can set the thermostat to 82 and still chill out.

Advocates toss around energy-savings figures - I've seen estimates from a fan company as high as 40 percent savings in summer. The truth is, it depends mightily on all sorts of factors - about your house, your comfort zone, and the temperature outside.

"One thing that's for sure is an air-conditioner uses a lot more energy than a ceiling fan," which generally uses the same as a 100-watt lightbulb, says Joe Rey-Barreau, a professor and lighting designer at the University of Kentucky. So the less you can use your air-conditioner, the better. "That's the bottom line."

Note that the fan doesn't cool the room. (In fact, as University of South Florida researcher Muhammad Rahman found, it actually warms a room slightly because of heat from the motor.)

But it makes people feel cooler because of the wind-chill effect; it speeds evaporation from our 2.6 million sweat glands.

Thanks to my own ceiling fan, I've used the air-conditioner only one day this summer, though admittedly, it has been a cool one.

It's in a bedroom upstairs - a prime location in most homes, because second-floor rooms are often hotter. Plus, why supercool the entire downstairs all night when the fan will suffice?

A few tricks will help increase the savings: Turn the fan off when you leave the room. Otherwise, you're wasting energy, while heating the room to boot.

If the fan also has a light fixture - another heat source - turn the lights off.

And in winter, oddly enough, leave the fan on!

Just reverse the flow. (Look for a switch on the side of the motor housing.) In summer, the fan should blow down. In winter, it should blow up. The air will circulate across the ceiling to the edge of the room and down the walls, mixing with the cooler air below.

Just how much temperature stratification there is depends on the room and a lot of other factors. But I gather from several sources that 10 to 20 degrees of difference isn't uncommon.

Generally, you get what you pay for in a fan. But whatever your budget, opt for an Energy Star fan, which is about 20 percent more efficient than others. It's a matter of blade design and improved motors.

Then think of Rohles on the next summer day when the fan is spinning, but the electric meter isn't.

Cool!

GreenSpace: Tips on Buying a Ceiling Fan

For ceiling-fan basics and a list of fans - more than 1,400 - that qualify as Energy Star fans, visit www.energystar.gov (Under "Products," click on "Heating & Cooling," then "Ceiling Fans"). The site also lists the suggested fan size for a given room, explains mounting systems, and more.

Although not all fans list this information on the box, check for blade pitch. A pitch of about 16 degrees is considered optimal. Many fans with less pitch won't move as much air.

Check the motor size, expressed in mm. An example would be 188x25 mm. Higher numbers indicate a more powerful motor. It can be a deciding factor in two fans with the same blade pitch and diameter.

SOURCES: The federal government's Energy Star program, and Gary O'Connor

of Eagle Ener$ave, a fan store in Drexel Hill.

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com. To post a comment, visit her blog at http://go.philly.com/greenspace.