Monday, August 3, 2015

Apple #715: Fan on High vs. Fan on Low

Let me tell you a little story about fans.  Well, two stories, actually.  Maybe three, if you count the cows.

Recently, a friend of mine was visiting.  We'll call him Montgomery.  We went to an all-day event and when we came back to my house, we discovered the A/C was not working.  Broken. Busted.  No go, Charlie.  It had been in the 90s most of the day and very humid.  So it was really hot & stuffy in my house.

I got out one of my trusty box fans and Montgomery turned it on while I tried to see if the reason the A/C wasn't working was anything I could fix.  Turned out, no, the motor was dead.  Nothing I could do about that except put in a call for a repair man.  After doing that, I sat down on the couch in front of which Montgomery had placed the fan and turned it on.

This is very like the box fans I own.  Mine are also Lasko Weather Shield models.  They do the job, and they don't rattle. You can buy this fan from Amazon for about $39.  I got mine from Target for about $10 less, if memory serves.

Normally I turn my trusty box fans immediately to the highest possible speed (3 out of 3).  Montgomery had set it to 2.  I thought, of course at the higher speed the fan will cool things off faster, or more, or better.  So I turned the fan up to 3.  Much to my great surprise, the higher speed did not feel better.  In fact, it kind of felt worse.  Like it was blowing warmer air on us, or the higher speed air was battering us but not really cooling things off.  So I put it back down to 2, and instantly it felt better.  We felt more of a cooling effect, and the speed was gentler and more lulling.  We sat in front of the fan on medium speed and talked for a long time, and felt pretty comfortable, in spite of the hot stuffiness elsewhere in the house.

I vowed to remember this lesson and try it on my own another time.  So I did and now we come to my second story.  In both senses of that phrase.

My bedroom is upstairs, and the A/C even when it is working properly does a poorer job of cooling things off up there.  So usually at night I turn on one of my trusty box fans -- exact same kind as the one Montgomery & I had used -- aimed at where I sleep in the bed.  I keep the fan positioned a fair ways away from the bed, over by the closet, because that's where the A/C vent is.  My theory is the fan will blow the cool air that comes out of the A/C vent in my direction where I'm sleeping in bed.  It is this fan that I always turn up to 3.

I also put my box fan on a chair so it's roughly the same height as my bed.  Not as high as on the stool shown in this photo.  But a similar idea.
(Photo from Lennbob on Flickr)

But after Montgomery's visit, I tried out the fan at level 2 instead.  I thought that, as with my experience with Montgomery, the fan on the lower setting would feel cooler and more gentle and pleasant, and I would save energy besides.  So I lay in my bed, waiting to feel the gentle cooling of the medium setting.  Instead, I did not feel cooled off enough.  I tried this on three different occasions, and each time, I had to get up and turn the fan up to 3.  Then I could feel the cool breeze and drop off to sleep.

So what gives?  Why did the fan work better on level 2 in one instance, and level 3 in another?  You've probably already guessed the answer.  But first, let me tell you a few things I learned about fans.

  • The little truism people like to say is: Fans cool people, not rooms.  They churn the air, and the motion of the air over your skin causes the sweat your body produces to evaporate that much more quickly, and thus your body cools off more quickly as a result.  Fans, in essence, create wind chill.
  • So a fan will help you cool off more quickly.  But it doesn't do much to change the temperature of the room.
  • There's kind of an asterisk to that statement which is the case of ceiling fans.  Ceiling fans move the air just as a box fan does, but since it's situated on the ceiling and since cooler air wants to sink while hot air wants to rise, a ceiling fan will create an entire column of air that is about the height of the room.  You will feel the breeze of the moving column of air, but that column also puts in motion the hot air at the top of the room, causing it to cool slightly and descend.  So a ceiling fan does cool the air in a room a little bit.

How a ceiling fan distributes the air and therefore regulates the air temperature.
(Diagram from the Fan Doctor)

In the summer, turn on your ceiling fan so it rotates counter clockwise and pushes the air downward beneath the fan. In winter, reverse the fan so it rotates clockwise and moves air upward beneath the fan. 
(Diagram from the Fan Doctor)

  • Former stock car racer and inventor Walter Boyd got the idea to maximize this principle when he invented the High-Volume Low-Speed fan, specifically to keep dairy cattle cool.
  • He had invented all sorts of things, from modifications to improve the safety and performance of race cars to a machine that improves the way roofing materials are applied, to a machine that raises the Jumbotron.  So I guess somebody asked him, hey, guy who is good at inventing improvements to things, can you figure out a better way to keep my cows cool?
  • The reason this is important is because if dairy cows get too hot, they don't produce milk.  They are rather sensitive creatures, it turns out, and their threshold for temperature comfort is lower than people might have guessed.  If you're putting all your dairy cows in the milking barn and it's a warm day, the dairy cows are going to get too hot to give as much milk, or maybe some won't give any milk at all, and nobody's going to be very happy about that.
  • Farmers were using fans in their cattle barns but the fans weren't doing enough to keep the cows chill (no, regular reader Jason, that is not a typo. No need to correct me).  They needed a better solution that would not cost a lot.

A dairy cattle milk barn. Note the box fans suspended above the cows.
(Photo from Modern Farmer

  • So Boyd thought about fans.  He noticed that if a fan is on too high, sometimes that's not comfortable.  Sometimes the speed of the fan does not make you cooler.  So it probably wouldn't solve the problem to speed up the fans the dairy farmers were already using.
  • He also knew that these dairy barns are huge, and it's of little use to have a relatively small fan in front of only one or two cows.  What you really need is to get all the air in that huge cattle barn moving.  You don't have to get it moving fast, but you have to get it moving.  So he came up with a giant fan that would move a lot of air in a large room: the high-volume low-speed fan.

Example of a high-volume low-speed fan (HVLS).
(Photo from Nelson Equipment)

  • These HVLS fans are attached to the ceiling, so they're taking advantage of that principle of moving an entire column of air.  The blades of the fan are enormously long so they'll move a lot of air.  They're also modeled after airplane propellers, not only in the materials with which they're constructed but also in the sense that the angle at which the blades are tilted will maximize the amount of airflow their motion can generate.
  • These fans were regarded as a great innovation because they did help cool those dairy barns at relatively little cost.  So dairy farmers all over the place started buying and installing these things left & right.  If you've ever been in a great big barn-like room, you've probably seen one of these.  They are installed in most of the buildings in my state's fairgrounds, for example.  
  • The HVLS fans did improve things, but not quite enough, it turns out.  The dairy farmers conducted more research over the years and discovered if they attached water spray nozzles to the HVLS fans so the cows get sprayed with a fine mist of water while the fans move the air and therefore also distribute that mist, then the cows would stay cool enough even in very hot weather to keep producing milk.  
  • Hey, nobody wants an unhappy cow.

(Photo from Apple Guardians

  • But the point of this HVLS story is that this Boyd guy knew that low-speed fans can sometimes be more effective than a high-speed ones.
  • There are also various laws of aerodynamics that suggest that a low-speed fan may be more effective. For example:
    • In order to 2x the speed of a fan, you have to 8x the power of the motor.  A higher-powered motor will generate more heat by its action so now you have a high-speed fan pushing the hot air it's creating as well as the hot air in the room.
    • (I also read a lot of things about mass air flow vs. volumetric air flow, but I don't feel that I understand the difference well enough to explain it to you.  Suffice to say, it led me to the conclusion that moving a lot of air slowly could be more effective than moving a little air quickly.)
  • So why the heck, if all those things are true about slow-moving fans, wasn't my box fan just as effective or even more effective at the slower speed?
  • The answer, I think, is distance.  When Montgomery was here, we were sitting very close to the fan on level 2.  Like, maybe 2 feet away from it at most.  The fan I keep up in my room I have maybe 6 or 7 feet away from the bed.  If I'd moved it closer, the level 2 speed might have felt more comfortable than the level 3 speed.  
  • But since I want the fan to move the cool air from the A/C vent in my direction, and since the moving air has to travel a lot farther to accomplish that, I need the fan to have a higher speed so it can create more force to move the air farther.  
  • (I really hope my high school physics teacher, Mr. Wisz, would agree.  No joke, that was his name.  He was the best.)

Another demonstration of the fact that distance matters.  The ceiling fan people are saying that when a room's ceiling gets higher, and thus farther away from you, you don't want to get a ceiling fan that goes really fast to make the moving air travel farther. Instead, you make sure the fan is positioned closer to you so you can better feel its effects.
(Diagram from

  • But the point for all you fan-running folks out there is this: Don't necessarily assume that putting your fan on the highest possible speed will give you the best results in all situations.  There are lots of variables -- distance from you to the fan being one of them, but also the size of your fan, the level of humidity (air density), how much heat your fan creates in doing its job, and who knows what else.  The number of ice cream sundaes in the room.  
  • Try your fan out at a lower speed first.  Give it a few minutes, see if that's sufficient.  If it's not, then turn the thing up.  You might be surprised to find that the lower speed was better.  Or maybe not.

(Photo from FarmTek)

  • Bonus question: why are fans made to go OFF HI MED LOW as opposed to OFF LOW MED HI?
  • Answer: to overcome impedance (like resistance but in an electrical current), the voltage of the fan has to be high enough to get the thing going.  Then it can be turned to a lower setting.

P. S. Here's another trick I used to use in an apartment that did not have air conditioning: put a bowl of ice cubes in front of the fan. Here, they're using those freezer pack things.  Probably a better idea as they stay frozen longer.

(Photo from The Good Human)

Sources and Stuff I Read
Howstuffworks, How do fans make you feel cooler?
Mental Floss, How Does a Fan Work to Cool You Off?
Electronics Cooling, All You Need to Know about Fans, May 1, 1996
Lasko, Create a wind chill in your home with help from electric fans
SF Gate, The Direction & Speed of Ceiling Fan Rotation in the Winter & Summer
Spoke, Walter Boyd 
Architectural Record manufacturer-sponsored article, HVAC for Large Spaces: The Sustainable Benefits of HVLS Fans, December 2009
Plant Services, Walter Boyd talks about HVLS fan technology, March 25, 2011
Private University Products and News, Big Fans Improve Comfort and Efficiency across Campus
J. W. Worley and J. K. Bernard, Cooling Effectiveness of High-Volume Low-Speed Fans Versus Conventional Fans in a Free-Stall Dairy Barn in Hot, Humid Conditions, The Professional Animal Scientist, 24 (2008): 23-28 (pdf)
Wikipedia, High-volume low-speed fan
Hudson Products Corporation, The Basics of Axial Flow Fans (pdf)
Teledyne Hastings Instruments, Mass Flow versus Volumetric Flow
The Vacuum Lab, The Difference Between Volume Flow and Mass Flow (pdf)
Tangent Labs, Volumetric Flow vs. Mass Flow (pdf)
UC Boulder, Density, Mass Flow Rate, & Volumetric Flow Rate  (video)
Emerson Network Power, Achieving Energy-Efficient Data Center Cooling: Does Reducing Fan Speed Always Result in Energy Savings? (pdf)

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