7 Things You May Not Know About Ceiling Fans

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This fan should not be your first choice for keeping cool.  See point 4 below for the reason.

Here we are in the middle of air conditioning season.  So why don't we chop down some myths and misconceptions about ceiling fans.  What got me on to this topic was a video of a fan with blades that hide on top of the fan when the fan is turned off.  Sounds clever, but it's a ridiculous idea.

Anyway, here are seven things about ceiling fans that a lot of people seem not to know.

1.  Ceiling fans heat the room

Yes, a ceiling fan is a cooling device.  (See number 2 below.)  But its effect on the room it's in is to add heat.  Why?  Because electric motors are devices that turn electrical energy into mechanical energy, most of which ends up as heat.  The infrared image below shows a ceiling fan motor that's hotter than the room it's in.  From the second law of thermodynamics, we know where that heat is going — into the cooler room.

Infrared image showing a higher-temperature ceiling fan as the motor generates heat

The net result of running a ceiling fan is that you're adding heat to the room.

2.  Ceiling fans cool people

Ceiling fans are useful for cooling only when they move air over skin.  They cool our bodies two ways:  by aiding evaporative cooling and by aiding convective cooling.  If the air movement created by a ceiling fan isn't hitting anyone's skin, it's just making the space warmer with no cooling benefit.

3.  A fan's efficacy tells you how well it moves air

Every new ceiling fan being sold in the US these days is labeled with its efficacy.  (Efficacy is an efficiency rating where the output and input quantities have different units.)  For fans, the measure of efficacy is how much air flow you get for the amount of electrical energy you put in.  Its units are cubic feet per minute (cfm) of air flow per watt (W) of electrical power.  A good fan will give you more than 100 cfm per watt;  a poor one might be as low as 30 cfm per watt.

Ceiling fan efficacy label

Next time you're looking for a ceiling fan, check the label.

4.  Bigger is better with ceiling fans

While you're checking those labels, you may notice a correlation.  The fans with the longest blades have the highest efficacies and those with the shortest blades have the lowest.  That's why the company Big Ass Fans makes big ass fans.  And it's why you'll want to avoid the little short-blade fans like the one above, no matter how cute, if you're interested in air flow.  If you just want cute, though, go right ahead.

5.  Lower speeds are more efficient

Another thing you'll notice when looking at fan efficacy labels is that you'll get more cfm per watt when you run the fan on medium than on high and more still on low than on medium.  The only logical conclusion here is to get the biggest fan you can fit into the room, leaving proper clearances, and run it on the lowest speed that keeps you comfortable. 

That's why the Big Ass Fans company was originally called the HVLS Fan Company.  HVLS stands for high volume, low speed.

6.  Ceiling fans probably won't save you any money if you have air conditioning

Martin Holladay covered this in his ceiling fan article from 2010, but it's worth reviewing.  If you don't have air conditioning at all, having some kind of fans can preserve your sanity.  You get to keep cool for a relatively low cost. 

Once you have a house with air conditioning, though, the dynamics change.  That air moving over your skin still feels good, and so does the low temperature, low humidity air produced by the air conditioner.  The hypothesis is that people will raise the AC thermostat setting if they're feeling the breeze of the ceiling fan, but the data don't support it. 

In 1996, the Florida Solar Energy Center (FSEC) did a study of homes with ceiling fans. They found that even though the fans ran more than half the day in the test homes, they saw no difference in thermostat setpoints in homes with ceiling fans compared to homes without. 

In short, for ceiling fans to save you money on your energy bills, you have to set the thermostat to a higher temperature.  I do it in my house but FSEC found that most people don't.  Better yet, use the fans instead of air conditioning when you can.  Here in the Southeast, that's usually in the spring and fall because of the humidity thing we've got here.

7.  A ceiling fan can decapitate you

I didn't even know there was a myth about this until I saw the Myth Busters video below.  Apparently some people worry about getting their heads chopped off by a ceiling fan.  And it can certainly happen, as you'll see in the video below...but only if you replace the motor with a more powerful one (like a lawn mower motor) and change out the ceiling fan paddles with razor-sharp blades.

So relax!  You won't get your head chopped off by a (normal) ceiling fan.  But you can certainly use more energy and make your home warmer by using one.

Oh, and that fan with the nesting blades is a ridiculous idea because it has two problems:  The blades have to be short to be able to nest together on top of the motor, and the blades are designed for nesting, not moving air.  If you don't like the looks of a ceiling fan, that's fine.  But why even have something like this at all if it's not going to move much air?

 

Related Articles

With Ceiling Fans, Bigger Is Better

How Energy Efficient Is Your Ceiling Fan?

Turn Off Those Ceiling Fans!

What Is the Difference Between Energy Efficiency and Efficacy?

 

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Comments

What is convective cooling? Moving air increases evaporative cooling so what is the difference?

And has anyone else used such a silly captcha in this century?

Allison
Bailes

Lloyd, convective cooling is moving warmer air out and cooler air in.  When a breeze aids in evaporative cooling, it's moving humid air away and replacing it with drier air.  The former is sensible cooling since it lowers dry bulb temperature.  The latter is a form of latent cooling since it lowers the vapor pressure of the air near the skin, thereby allowing more water to evaporate from the skin.

I wish I could take credit for the captcha we use, but I can't.  I saw it somewhere else first and liked it much better than trying to figure out sometimes illegible characters.

Do you have any thoughts on ceiling fan on covered decks on speed and cooling and speed and insect repelling?

Allison
Bailes

K, yes, fans on a porch are great for cooling and also help keep bugs at bay.  As far as speed, just find what works.  The problem with fans on porches is that sometimes people leave them running all the time.  They may not be heating up the inside of the house, but they're not doing any good either.  As with indoor fans, turn them off when no one's there to benefit from their breezes.

Moving air is highly effective in repelling mosquitos. Not only does it "blow them off course" at higher speeds, but even more effectively is makes it impossible to follow your scent back to you. Search the NY Times for a detailed article on the research.

We have a gazebo on our back deck that includes a ceiling fan...and yes, it does add to the comfort and does help keep the bugs at bay. Historically, it is only turned on when we were outside. But this spring, we left it on during the day...as it was the only successful deterrent we found in keeping the pesky robin's from nesting in the gazebo! Worked like a charm!

I recommend ceiling fans mostly for the heating season in rooms with higher than 8 foot ceilings. My wife insists on using the ceiling fans in our vaulted ceiling family room during the cooling season. I cannot convince her that they just make the room warmer by pulling the warmer air near the ceiling down to where she is.

Allison
Bailes

Robin, there's no evidence that running ceiling fans in reverse in the winter saves energy.  Here's what Martin Holladay wrote about that in his ceiling fan article:

“The only problem with the claim is that no researcher has ever been able to confirm it. The ‘destratification savings’ myth was debunked 17 years ago, when an article in the June 1993 issue of Consumer Reports pointed out that there is no evidence to back the claim that destratification saves energy.”

and

"If you’ve been operating your ceiling fans during the winter, run this experiment: turn off the fans and see if you are more comfortable or less comfortable. If you actually need to run the fan to feel comfortable, something is seriously wrong with your heat distribution system or the integrity of your home’s thermal envelope. Instead of using electricity to operate your ceiling fans during the winter, it’s time to diagnose the source of the comfort problem — perhaps poorly located registers or a leaky ceiling — and solve the actual problem.”

Actually, I had the same question as Robin, but I don't think it was addressed in any responses so far. This comes up at a club I belong to with our outdoor patio. People keep climbing up and reversing the fans during the summer, since "it's just blowing hot air on you." I believe I actually take the opposite position as Robin on this and figured till reading that comment that this article finally settled this. i.e. It's the airflow that's cooling you and if you're getting more airflow from the fans pointed downward, then that's better. There might be some somewhat hotter air at the top of our outdoor canopy, but I figured that was more than offset. Who knows, maybe we need a psychologist on the panel to figure out the real solution for either side on this one. #smallthingsthatdrivepeoplebonkers

Dr. Allison,
Please call my wife and tell her that I have, and continue to be, correct in saying, "Sweetheart, fans cool people, not rooms and objects." I'll ask her to read your post but I suspect that may not be enough to convert her into being a believer.
john

Allison
Bailes

John, what I've come to realize is that harmony at home sometimes calls for compromising on building science issues.  I've even given up trying to get my wife on the Celsius temperature scale.

John,
Perhaps your wife can understand that evaporative cooling takes place regardless of the direction of airflow, whether it is up, down or from the side. If you can actually feel the airflow from a downward flow then it is definitely cooling you and you can test this by siting directly under the fan in the winter. I think this argument is easier to prove than the reason my wife likes to leave the fan on regardless of the season, which is to keep rooms, and "energy", from being stagnant. Airflow just makes the room feel "more energetic". Good luck!

John, and others worried about your wife leaving you since you will not quit ranting about the ceiling fan. I looked up one of the Big Ass Fans and it draws about 3 watts on the lowest speed. Another much less expensive and popular brand draws about 10 watts on low speed. Assuming you can keep this marriage buster on low speed and the lights turned off, you can run one of those things for $.25 to $.75 per month. If I hear about this thing breaking up any relationships, I will then know the value of the union.

Dan, I'm not worried about her leaving me over this. The crap she put up with for the past 42 years has dulled her ambition; I simply will receive "That Look."

Here's another thing...recently learned that if you're in a commercial or industrial building that the fan must turn off during a fire sprinkler release since they disrupt the coverage pattern. Also some studies show that sprinkler heads can have a delayed reaction due to the cooling effect at the ceiling.

Allison
Bailes

I wasn't aware of that but it makes sense.  Thanks, Lee!

Fans retard the reaction time of smoke alarms too.

But are they evil?

Allison
Bailes

Well, Carl, now that you mention it, I do recall seeing an article titled, Ceiling Fans Are Evil.  But I don't think they quite reach the level of electric furnaces, unvented gas logs, or Walter White on the evil scale. 

Oh, wait.  That article was by you!

How long has the efficacy number been around?

Allison
Bailes

Good question, John.  I first noticed the labels in 2012 and I'm pretty sure they were there when I bought ceiling fans for the house I built in 2003.  So it was sometime in that 9 year window.

When two bodies have different cooling and heating needs, a ceiling fan can be a great equalizer. I sleep under the covers, where the fan provides no cooling to my delight, while she can unwrap as needed to get the latent heat relief she needs a times during the night.

Now, if I could only find some "efficient" fans that fit a Tuscan decor. Industrial does not quite fit the bill.

At night, I absolutely can and do set my stat a couple of degrees higher with a ceiling fan running than without one (as when I travel). Just because "most people" don't (as that limited study would have us believe) doesn't mean it can't be true for anyone.

The problem with Martin's position (as evidenced by the quotes, I can't recall the entirity of his article) is that he would discourage anyone from using a fan to good effect.

It should be pointed out what should be obvious to any energy efficiency pro is that high efficiency fans produce proportionally less heat. I would argue that running an Energy Star rated fan on low or even medium speed coupled with any change in the t'stat is a no-brainer from an EE standpoint.

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