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Building Science

The Important Stuff You Need to Know About Ceiling Fans

Here are a few basic principles that many people don’t understand

Image 1 of 3
This ceiling fan with short blades may be cute, but it's not your best choice if you want a fan that moves air.
Image Credit: Alexander Bell
This ceiling fan with short blades may be cute, but it's not your best choice if you want a fan that moves air.
Image Credit: Alexander Bell
An infrared image of a ceiling fan during operation. Notice that the motor is warmer than the room.
Image Credit: Alexander Bell
A ceiling fan efficacy label showing one that moves 106 cfm for each watt of electricity
Image Credit: Energy Vanguard

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 point 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 photo reproduced as Image #2 (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.

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.

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, no matter how cute, if you’re interested in air flow. If you just want cute, though, go 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 Fan 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.

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 blades with a razor-sharp variety.

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?

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

9 Comments

  1. PioneerBuilders | | #1

    Ceiling fan in a large vault
    We regularly construct homes with large vaults in the great room, dining room, kitchen areas, and we are 100% using mini-split systems. One concerns we've had is the area in the vault getting stale, so we've installed ceiling fans to move the air around. Any thoughts on this? In other words, it isn't about the air touching the skin for evaporative/conductive effect, just air sterility (if that's a word that can be applied here).

  2. Steve Knapp CZ 3A Georgia | | #2

    @PioneerBuilders
    How do you ventilate your new homes? A good strategy should address any concerns about stale air.

  3. Trevor Lambert | | #3

    air sterlility
    No, that is definitely not a word to be applied here. Sterility has to do with the absence of living organisms. Air sterility is not really a goal, but even if it was, you're not going anywhere towards achieving it by moving air around. I think the concern about stale air due to high vaulted ceilings is without merit. The amount of air movement required in order for it to be relatively mixed is rather small, such that convection currents will likely be sufficient. Even just walking around in the room would have a significant effect.

  4. Trevor Lambert | | #4

    fan with nesting blades
    What would be an example of a fan with nesting blades? A Dyson fan? I tried searching the term and came up with nothing besides birds nesting on fan blades.

  5. User avater
    Dana Dorsett | | #5

    Is stale air even a problem for a vaulted ceiling?
    The relative humidity (RH) of the hot air in the vaulted ceiling is lower than the cooler stratified air near the floor. Mixing it up increases the cooling load by raising the temperature near the floor where the humans are.

    What problems are being avoided by circulating/purging that warm stagnating air near the ceiling?

  6. User avater GBA Editor
    Allison A. Bailes III, PhD | | #6

    Nesting blade fan
    Trevor, here's a video of the fan I mentioned:

    https://youtu.be/T0tGn8MjJn0

  7. Stephen Cook | | #7

    Lets give a shout out to the newer DC motor ceiling fans that are available. The Emerson ECO line, as one example, is as high as high as 355 CFM/Watt which is quite high for a ceiling fan.

    Its good to have alternatives to the standard K55 stack (my old favorite) or 188 pancake motors. The DC motors are more expensive due to the electronics required, but silent running and efficiency are some of the benefits.

  8. user-6895662 | | #8

    Sure, ceiling fans heat the room, but not by much. If a fan draws 50W and is 80% efficient, that's 10W dissipated as heat, or 34 BTU/hr. Not a big deal as long as the fan is being used to cool people or pets, as you noted.

  9. User avater GBA Editor
    Martin Holladay | | #9

    User 6895662,
    Actually, all of the electrical current drawn by an appliance degrades to heat -- and if the appliance is indoors, the appliance is heating your home. If it's a 50-watt fan, it is adding 50 watts of heat, not 10 watts of heat, to the room whenever it is turned on.

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