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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

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

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…

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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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