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Physics of a reversible ceiling fan

  1. Jun 13, 2005 #1

    DaveC426913

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    I often hear people suggest switching the direction of a ceiling fan from 'blow' to 'suck' or the reverse, thinking that one will draw cool air in better.

    My intuition tells me that:
    - a ceiling fan blows air in a doughnut patterrn, where air flows either up in the centre or down in the centre, and that
    - both directions are equally effective (or ineffective) in circulating air in the room, and thus circulating air to outside and from outside the room too.
    - the only real difference between the two directions on a ceiling fan is whether or not you feel the breeze (you will feel a breeze in the room if the fan is switched to downwards, the breeze in the room will be much less noticeable if the fan is switched to upwards)
    - that the idea behind a reversible fan is merely one of whether or not you want to feel a breeze or not (eg. you might not want a breeze when operating it in the winter, thus you would switch it to upwards)


    In short, I do not believe that reversing the direction of a ceiling fan will have any effect on its circulating ability and thus its cooling ability.

    Can someone clarify?
     
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  3. Jun 13, 2005 #2

    Evo

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    It's not to do with cooling. Supposedly running the fan in reverse will send warm air near the ceiling down into the room, you are supposed to do this in the winter. This makes more sense if you have a high ceiling.
     
  4. Jun 13, 2005 #3
    That is a great question, something I have always wondered too. Personally I do not think it makes a difference either, but I am not a physicist, or fan expert.

    *pulling from other thread* Shouldn't this go in the fan section? Here are some links to some nice fans. www.fans.com www.coolfans.net (note: I am unsure of whether or not these are real links :smile: )

    edit... Evo could be correct:

    From: http://en.wikipedia.org/wiki/Ceiling_fan#Ceiling_fan
    It talks about the general principals; however, it does not say anything about direction.
     
    Last edited: Jun 13, 2005
  5. Jun 13, 2005 #4
    I always thought the reverse was just to keep air circulating, rather than to cool or heat it.

    Heres an interesting question, the ceiling fan in our veranda (sp) in the back yard is rarely used, and it has much saggier edges than the ones in our home, whats the culprit? It is a wood fan.
     
  6. Jun 13, 2005 #5

    Janus

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    Or if you have that brain-dead invention called "ceiling heat".
     
  7. Jun 13, 2005 #6

    Moonbear

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    I have 1) never felt any difference either way I run a fan, not even in creating a draft and 2) can't figure out why it would matter whether you run the fan forward or reverse to disperse heat from the ceiling. In reverse, you pull air from the lower part of the room, up toward the ceiling, which then pushes the warmer air near the ceiling down into the room. In forward, you pull air from the warmer area near the ceiling and push it down toward the lower part of the room. Aren't both directions serving the same purpose of keeping the air in the room circulating from ceiling to floor and vice versa? To me, the only difference between summer and winter is whether you have the windows open in summer, which means circulating outside air into the room as well as just keeping the room air circulating to keep the heating more uniform.

    I actually just turned on my kitchen fan for the first time since last summer two days ago, so can report that the first thing I felt as I turned it on was a blast of hot air from the ceiling until the room air equilibrated a bit. I think it really serves to keep my second floor cooler by not allowing the hot air to accumulate near the ceiling of the first floor to transfer the heat to the second floor.
     
  8. Jun 13, 2005 #7
    hmm, seeing as this is a serious thread, maybe it should be in the physics forum?

    at any rate, thought maybe it was worth mentioning that window fans also have "suck" vs "blow" options. apparently to either suck cold air into the house, or to blow the hot air out? i would assume its the same idea though... with the fan blowing air in certain directions i mean. Cept, i think with the winow it makes a little more sense maybe? or maybe not, i dunno.
     
  9. Jun 13, 2005 #8

    Moonbear

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    Yeah, it makes a lot more sense to me with a window fan to either suck the hot air out of a room, or blow outside air into it. I actually prefer two window fans, one sucking and one blowing (on different windows of course :rolleyes:).
     
  10. Jun 13, 2005 #9

    Evo

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    You doubted me? :grumpy: I just happened to remember that was always what the ceiling fan manufacturers claimed, that they were useful year 'round because of the "reverse" for winter use. :rolleyes:
     
  11. Jun 13, 2005 #10

    Evo

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    The logic is that in reverse, you are circulating the warmer air from near the ceiling down into the room without creating a direct draft that would make things feel cooler. In forward, even though it's still stirring up the warmer air, the breeze it is creating should more than compensate. You must not be running your fan very high if you can't feel the wind chill factor. :bugeye:
     
  12. Jun 13, 2005 #11

    Janus

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    the difference I can see is this:
    When the fan is blowing down, it is only "sucking" air from the area immediately above the fan. Now this air will be replaced, but how much will be from the rest of the hot air near ceiling? The air blown down will lose speed as it colides with the air mass below and a portion of it will travel back up to the area near the fan to be recycled. By the time the downward flow hitts the floor it will be fairly weak and there won't be much side deflection to genrate a side flow.

    In the reverse position, the air sucks form just below the fan and blows against the ceiling. Most fans are much closer to the ceiling than the floor so you are going to get a stronger blast hitting the ceiling and thus a greater sideways deflection pushing the warm air near the ceiling out of the way and mixing it better with the other air in the room.

    I agree that if the the fan were located an equal distance from floor and ceiling running either way gives you an equal amount of air mixing.
     
  13. Jun 13, 2005 #12

    Moonbear

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    I can feel the wind, but I also feel it when it's in reverse. I thought it made the room too cold in the winter. Maybe if you have a huge room and a small fan you won't notice the draft in reverse???
     
  14. Jun 13, 2005 #13

    Evo

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    The trick is to run the fan very slowly when in reverse.

    We could never get married. :frown: I like it cold especially when I sleep and I have a fan on high all year, I need the feeling of wind in my face or I feel suffocated.
     
  15. Jun 13, 2005 #14

    BobG

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    I wouldn't pay much attention to the printed instructions - they haven't seen your house. It really depends on where the fan is located and the characteristics of the room. If it's located where it can suck the cool air up out of the basement (or from a lower floor into an attic), and the windows are open, you do get more than just recirculation. Unless you're trying to draw cool air up from a lower level, I think you just have to try both settings to see which one draws more cool air in the window.
     
  16. Jun 13, 2005 #15

    Evo

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    A ceiling fan?
     
  17. Jun 13, 2005 #16

    Moonbear

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    No offense, but I'd rather marry a man anyway. :tongue2: Yeah, you'd probably die in my house. One of my friends is like you, and I have to wrap myself in a blanket even in the summer when I visit her because she keeps the house so cold, and she practically suffocates in my house. My thermostat currently reads 84 degrees, and I'm comfy. Even with the heat up to 72 in the winter, I still have to bundle up in sweaters and curl up with a blanket...that's cold for me. My mom used to keep the temperature set for somewhere between 68 and 70 when I was growing up, and I'd wear gloves inside and sit next to the heating vent to stay warm (good thing we didn't have cats, or I'd be fighting a cat for the warm, sunny spots to sit in).

    I do know what the manufacturers say about their fans, and it is the same thing you say, but it's not how it feels to me. Then again, I don't have any huge rooms or high ceilings, so it might help there to keep all that heat from being trapped way up high.
     
  18. Jun 14, 2005 #17

    BobG

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    Yeah. We have one over the front stairs (we have a garden level basement, so the front door is on the landing between floors). That one draws air up from the basement (you'd need a tall step ladder to reverse the switch).

    Even in normal locations, it changes the air flow in the room including how much air moves in or out the windows. If the fan is blowing down, cool air coming in the window might flow towards the ceiling and then get cycled down through the fan before it's warmed much. Or, a fan blowing up might set up air currents that move air between the sunny side of the house and the shady side of the house. It's hard to know, since it depends on the lay out of the house and every piece of furniture you have disrupts the air currents and changes the flow.

    A window fan would be pretty predictable - the predominant direction of the air flow is horizontal, above most the obstructions.
     
  19. Jun 14, 2005 #18

    Chi Meson

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    We have a "vaulted ceiling" in a room that also has a wood stove. In the winter we keep the ceiling fan "blowing down" for two reasons: in this setting it pulls the cold ground are up past the wood stove, more efficiently getting the heat out of itl; second, when it is in "suck up" setting, it pulls cold ground air right past the sofa where we all like to sit.

    The owner's manual basically says to operate it just as Dave's OP states (for the same reasons). Ultimately, the setting to use is the one that's more comfortable.
     
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