Ceiling Fan Direction

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gorcee
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Ceiling Fan Direction

Postby gorcee » Sat Sep 25, 2010 2:04 am UTC

I was contemplating the conventional wisdom regarding ceiling fans today. The typical argument goes something like this: 1.) ceiling fans can't cool a room, 2.) in the summer, a ceiling fan should go anti-clockwise, or blow down, to create a draft, 3.) in winter a ceiling fan should suck up and be set clockwise at low speed, 4.) a ceiling fan will, at best, set the room temp to an equilibrium.

I contend that this is all bullshit. First, the equilibrium argument is valid only in a closed system. Second, in a building with forced air, you should reverse the conventional directions. So in the summer, the fan will pull cool air up from the floor, and disperse warm air about the rest of the ceiling. It is relevant to note that the conventional view shows that the ceiling fan induces a local vorticity in a vertical column centered on the fan. I see no reason for this to be true.

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Re: Ceiling Fan Direction

Postby gorcee » Sat Sep 25, 2010 2:07 am UTC

Sorry for double posting, my iPhone was bugging on the post length. Anyhow, I contend that in a building with central A/C you would want to be pulling up as much cool air from the floor as possible. Am I crazy? I think that using a fan in this manner you can save A/C costs, but have only anecdotal evidence to go on.

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Re: Ceiling Fan Direction

Postby badwiz » Sat Sep 25, 2010 3:21 am UTC

I think the answer to your question is more biological than thermo / fluid dynamical:

1. Your skin helps keep you cool by evaporation. Moving air on your skin, even if that air is warm, will cool you by amplifying evaporation.

2. Air is a crappy conductor of heat. If you sitting on a hot summer day and the air around your skin is hot, you are basically wrapping yourself in a warm blanket. Get new air to your body by moving it with a fan.

The above points seem to suggest to me, and also from experience, that its less about the relative air temperatures in a given room and more about helping your body naturally cool itself. Thus I would vote in favor of using a ceiling fan conventionally in the summer.

I cannot give a solid argument for reversing a ceiling fan direction in the winter; the primary concern with heating a house is heat lost through doors and windows.

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Re: Ceiling Fan Direction

Postby Charlie! » Sat Sep 25, 2010 6:17 pm UTC

I can't think of any reason why you would want the fan blowing any way but down. Down is much more efficient at creating wind near people-level, which is why you'd use a fan. Why would you want a ceiling fan going in the winter, anyways? I like my blanket of warm air in the winter.

Sure, in some cases fans will increase the heat transfer out of a room. But in a closed room that is near equilibrium with the outside (i.e. not 110 degrees inside on a 80 degree day), ceiling fans will make the room hotter.
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Re: Ceiling Fan Direction

Postby gorcee » Sat Sep 25, 2010 6:55 pm UTC

The conventional wisdom says that by keeping a ceiling fan on the lowest setting, sucking up, you help get the warm air off the ceiling and down into the room without causing a noticeable breeze.

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Re: Ceiling Fan Direction

Postby Charlie! » Sat Sep 25, 2010 8:00 pm UTC

gorcee wrote:The conventional wisdom says that by keeping a ceiling fan on the lowest setting, sucking up, you help get the warm air off the ceiling and down into the room without causing a noticeable breeze.

Moving air without causing a breeze would be a bit of a trick. It might be good in some high rooms with no internal breezes, where people couldn't stand still for long. You still wouldn't need to reverse the direction, though.
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Re: Ceiling Fan Direction

Postby Xanthir » Tue Sep 28, 2010 12:30 am UTC

Charlie! wrote:
gorcee wrote:The conventional wisdom says that by keeping a ceiling fan on the lowest setting, sucking up, you help get the warm air off the ceiling and down into the room without causing a noticeable breeze.

Moving air without causing a breeze would be a bit of a trick. It might be good in some high rooms with no internal breezes, where people couldn't stand still for long. You still wouldn't need to reverse the direction, though.

No noticeable breeze, not no breeze. The idea is that the fan produces a force over a smallish area, so you point that up away from people. The air that is then forced down comes down around the edges of the room, over a substantially larger area, so the flux at ground level is much smaller.
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Re: Ceiling Fan Direction

Postby gorcee » Tue Sep 28, 2010 5:53 pm UTC

Anyhow, all of these arguments seem to be re: passing air over people's skin.

My original question still stands: in an environment that is not closed (ie, forced air, open windows, multiple stories, etc.), is reversing the conventional wisdom a sensible way to help actually maintain a cooler/warmer temperature in a room?

For example, I have forced air vents in the floor of my upstairs bedrooms. The master bedroom has a ceiling fan. If I turn the fan on high, and have it suck air up, I contend that it helps get the cool air off the floor. Otherwise, there is a noticeable temperature gradient, and the cool air never gets to the thermostat, so the A/C runs too much. Anecdotally (is this a word?), I feel more comfortable with the fan sucking up in the summer. Am I just weird?

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Re: Ceiling Fan Direction

Postby Charlie! » Tue Sep 28, 2010 7:37 pm UTC

Xanthir wrote:The idea is that the fan produces a force over a smallish area, so you point that up away from people. The air that is then forced down comes down around the edges of the room, over a substantially larger area, so the flux at ground level is much smaller.

But if the flux at ground level is much smaller, then it's not doing the job you want it to do, which is mix the air throughout the room! It's simply more efficient to run the fan blowing downwards at a lower setting.


The reason making the fan "suck up" isn't any better at mixing in the cold air form the floor is because it isn't "lifting the air up." It's just making it move in a circle through the room. What goes up must come down. And, in this case, what goes down must come up - a fan blowing down onto the floor forces cold air up around the edges.

Regarding your anecdote, I think you should test it first :P Put some thermometers around clamped so they're mostly measuring the air, record them under different conditions. There are too many complications in this sort of thing if you don't test the thing you want directly.
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Re: Ceiling Fan Direction

Postby gorcee » Tue Sep 28, 2010 7:57 pm UTC

Charlie! wrote:
Xanthir wrote:The idea is that the fan produces a force over a smallish area, so you point that up away from people. The air that is then forced down comes down around the edges of the room, over a substantially larger area, so the flux at ground level is much smaller.

But if the flux at ground level is much smaller, then it's not doing the job you want it to do, which is mix the air throughout the room! It's simply more efficient to run the fan blowing downwards at a lower setting.


The reason making the fan "suck up" isn't any better at mixing in the cold air form the floor is because it isn't "lifting the air up." It's just making it move in a circle through the room. What goes up must come down. And, in this case, what goes down must come up - a fan blowing down onto the floor forces cold air up around the edges.

Regarding your anecdote, I think you should test it first :P Put some thermometers around clamped so they're mostly measuring the air, record them under different conditions. There are too many complications in this sort of thing if you don't test the thing you want directly.


I might do this next summer, if the heat is really as bad as it was this summer.

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Re: Ceiling Fan Direction

Postby Xanthir » Tue Sep 28, 2010 11:16 pm UTC

Charlie! wrote:
Xanthir wrote:The idea is that the fan produces a force over a smallish area, so you point that up away from people. The air that is then forced down comes down around the edges of the room, over a substantially larger area, so the flux at ground level is much smaller.

But if the flux at ground level is much smaller, then it's not doing the job you want it to do, which is mix the air throughout the room! It's simply more efficient to run the fan blowing downwards at a lower setting.

Sorry, I meant flux through the area above the fan blades, compared to a comparably sized area at ground level. It seems you may be thinking that I'm taking the "unit area" to be the size of the room itself, which would indeed be a problem (it would indicate an incomplete mixing, as you say, with some of the air cycling back through the fan quickly, rather than circulating through the room first).


The reason making the fan "suck up" isn't any better at mixing in the cold air form the floor is because it isn't "lifting the air up." It's just making it move in a circle through the room. What goes up must come down. And, in this case, what goes down must come up - a fan blowing down onto the floor forces cold air up around the edges.

Right, in pure mixing terms the two should be equivalent, modulo flow effects caused by room shape. But making the fan blow upwards accomplishes the mixing without causing any localized areas of high air flux near ground level, which feels cool to our skin. Thus it's better in winter, where feeling cool is counterproductive.


Edit: Let me be more explicit.

In an ideal room with an ideal fan at the center, running the fan produces the classic stable loop we see in convection diagrams in school. Around the edges of the room the flux is low; the air is moving, but not quickly enough for us to really notice it. Immediately around the fan, though, the flux is much higher. In the direction the fan is pointing, the high-flux region is shaped like a cylinder extending from the fan's face, gradually widening as it gets further out. In the opposite direction, the high-flux region is shaped roughly like a half-sphere, because it's generated by air being pulled into a low-pressure zone. Since a half-sphere's surface area grows much faster with increasing distance from the fan than a cylinder with the same radius does, the flux over the surface of the sphere at any given distance must be lower (since the total flux in front of the fan must be equal to the flux behind the fan).

High-flex regions of air are perceived as cooler to our skin because of the increased evaporation. Thus, in the summer you want to run the ideal fan downwards, and in the winter you want to run it upwards, so that the air you feel at the ground is comparatively cooler or warmer at the appropriate time.

Actual flow patterns in a room that disturb the air flow from an ideal mixing pattern may affect the relevance of this effect. In some rooms, running the fan up or down may mix the air better, thus introducing an additional variable into the "how cool/warm is the room at person level" question, which may be large enough to overwhelm the flux-induced variation in some circumstances.
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Re: Ceiling Fan Direction

Postby Charlie! » Wed Sep 29, 2010 1:39 am UTC

The problem is that although a fan blowing upwards will have less flux (breeze) at a point near the floor underneath, that is at the cost of effectiveness. You might say "well, it has less breeze at ground level than a down-blowing fan, therefore it's better." But the problem is that if your goal is to mix the air between the floor and ceiling, less breeze at ground level translates directly into less effectiveness. Breeze that never goes low is mostly wasted. The only way to reduce the breeze per mixing would be to increase the fan area or to use multiple coordinated fans.
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Re: Ceiling Fan Direction

Postby Xanthir » Wed Sep 29, 2010 2:57 am UTC

Charlie! wrote:The problem is that although a fan blowing upwards will have less flux (breeze) at a point near the floor underneath, that is at the cost of effectiveness. You might say "well, it has less breeze at ground level than a down-blowing fan, therefore it's better." But the problem is that if your goal is to mix the air between the floor and ceiling, less breeze at ground level translates directly into less effectiveness. Breeze that never goes low is mostly wasted. The only way to reduce the breeze per mixing would be to increase the fan area or to use multiple coordinated fans.

I think you're assuming that the fan produces more mixing on the half of the room that it points towards, and thus pointing it upwards results in less mixing at ground level, where you care about it. I don't know if this is an accurate assumption.
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Re: Ceiling Fan Direction

Postby tckthomas » Wed Sep 29, 2010 11:19 am UTC

Charlie! wrote:
Xanthir wrote:The idea is that the fan produces a force over a smallish area, so you point that up away from people. The air that is then forced down comes down around the edges of the room, over a substantially larger area, so the flux at ground level is much smaller.

But if the flux at ground level is much smaller, then it's not doing the job you want it to do, which is mix the air throughout the room! It's simply more efficient to run the fan blowing downwards at a lower setting.


The reason making the fan "suck up" isn't any better at mixing in the cold air form the floor is because it isn't "lifting the air up." It's just making it move in a circle through the room. What goes up must come down. And, in this case, what goes down must come up - a fan blowing down onto the floor forces cold air up around the edges.

Regarding your anecdote, I think you should test it first :P Put some thermometers around clamped so they're mostly measuring the air, record them under different conditions. There are too many complications in this sort of thing if you don't test the thing you want directly.


I haven't read all the posts, but still

No, you should put WET thermometers around. you want to approximate human skin. make a devious device that wets the thermometers (spray gun that sprays when you pull a lever connected to strings and gears?

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Re: Ceiling Fan Direction

Postby Charlie! » Wed Sep 29, 2010 12:28 pm UTC

Xanthir wrote:
Charlie! wrote:The problem is that although a fan blowing upwards will have less flux (breeze) at a point near the floor underneath, that is at the cost of effectiveness. You might say "well, it has less breeze at ground level than a down-blowing fan, therefore it's better." But the problem is that if your goal is to mix the air between the floor and ceiling, less breeze at ground level translates directly into less effectiveness. Breeze that never goes low is mostly wasted. The only way to reduce the breeze per mixing would be to increase the fan area or to use multiple coordinated fans.

I think you're assuming that the fan produces more mixing on the half of the room that it points towards, and thus pointing it upwards results in less mixing at ground level, where you care about it. I don't know if this is an accurate assumption.

It's equivalent to the assumption that the fan blowing down creates more breeze in the middle at human-height, which you yourself made :P

But yeah, for sufficiently slow fan speeds I agree the difference seems to go away. At regular ol' fan speeds the difference is just an empirical fact.
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Re: Ceiling Fan Direction

Postby gorcee » Wed Sep 29, 2010 3:35 pm UTC

Along the lines of the convection argument, this is what the Internets says happens (suggested Winter setting):

Image

I don't believe this to necessarily be true. This is what happens when a helicopter lands:

Image

The dust blows out, not up and around. Some blows up and around, but mostly it goes out. Now, the downwash from the rotors is of sufficiently higher pressure than the thermal gradient, but I also believe that a ceiling fan on high, blowing up, will have a downwash (in the direction of the ceiling) sufficiently higher than the thermal gradient. So this convective loop will be spread among a much larger portion of the room.

I think I will perform an experiment. Anyone have some good recommendations for cheap, easily mountable thermometers?

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Re: Ceiling Fan Direction

Postby Charlie! » Wed Sep 29, 2010 4:27 pm UTC

That's because helicopters aren't landing inside a room. If the wind tried to do that, it'd blow your walls down!
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Re: Ceiling Fan Direction

Postby sophyturtle » Wed Sep 29, 2010 4:53 pm UTC

I have noticed that the air blowing down feels cooler, and in the winter switched the fan to sucking up. We had high ceilings.

Mostly having anything moving the air and pushing the hot air that is at the ceiling around seems to help in the winter. I have no data, sorry about that.
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Re: Ceiling Fan Direction

Postby SWGlassPit » Wed Sep 29, 2010 5:09 pm UTC

Just to repeat what's already been said, blowing both up and down serve to break up any thermal stratification in the room, but having the fan blow down creates a direct, noticeable breeze that helps evaporative cooling from the skin, while the air movement at ground level when the fan blows up is far more diffuse.
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Re: Ceiling Fan Direction

Postby zlreitz » Thu Sep 30, 2010 2:16 am UTC

Whenever I use my fan (which is only during the summer) I always have it blowing upwards. Both my desk and my bed are against the walls of my room, and I don't get any noticeable difference with the fan blowing down. However, when it's pulling the air up, I can feel the breeze moving down the walls.

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Re: Ceiling Fan Direction

Postby gmalivuk » Thu Sep 30, 2010 3:35 pm UTC

Charlie! wrote:
Xanthir wrote:I think you're assuming that the fan produces more mixing on the half of the room that it points towards
It's equivalent to the assumption that the fan blowing down creates more breeze in the middle at human-height
How are these equivalent assumptions?
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Re: Ceiling Fan Direction

Postby Charlie! » Thu Sep 30, 2010 6:06 pm UTC

gmalivuk wrote:
Charlie! wrote:
Xanthir wrote:I think you're assuming that the fan produces more mixing on the half of the room that it points towards
It's equivalent to the assumption that the fan blowing down creates more breeze in the middle at human-height
How are these equivalent assumptions?

Assuming the fan isn't on fast enough to create big eddies, the breeze at ground level is directly related to the amount of air from ground level mixed with the rest of the room, per unit time.

It's basically the statement that you can't mix things without moving them. Which is only true if you're looking at timescales faster than diffusion, but we are, so yay.
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Re: Ceiling Fan Direction

Postby gmalivuk » Thu Sep 30, 2010 9:08 pm UTC

But what we're talking about is the breeze at one specific point at ground level (directly under the fan) versus the whole of the floor at ground level (if the fan's blowing upward). Mixing could be the same overall in both cases, even though there's clearly more of a breeze directly under a downward-pointing fan.
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Re: Ceiling Fan Direction

Postby Charlie! » Thu Sep 30, 2010 10:40 pm UTC

gmalivuk wrote:But what we're talking about is the breeze at one specific point at ground level (directly under the fan) versus the whole of the floor at ground level (if the fan's blowing upward). Mixing could be the same overall in both cases, even though there's clearly more of a breeze directly under a downward-pointing fan.

You can't have breeze horizontally along the bottom, obeying symmetry, without having more wind at the center. And you can't have wind vertically because there's a floor there!

Of course you could reduce the wind at one point by increasing the size of the dead spot at the center (basically what happens with sucking up), but then, well, you have a bigger dead spot at the center. Put another way, it means that fewer of the "flux lines" get close to the floor, and circulate though the middle of the room instead. You CAN have the same mixing (maybe defined in terms of the time it takes to move half the air on the floor off of the floor?), but it would mean turning up the fan, which is probably bad.
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Re: Ceiling Fan Direction

Postby ericgrau » Fri Oct 01, 2010 7:03 am UTC

gorcee wrote:Anyhow, all of these arguments seem to be re: passing air over people's skin.

My original question still stands: in an environment that is not closed (ie, forced air, open windows, multiple stories, etc.), is reversing the conventional wisdom a sensible way to help actually maintain a cooler/warmer temperature in a room?

For example, I have forced air vents in the floor of my upstairs bedrooms. The master bedroom has a ceiling fan. If I turn the fan on high, and have it suck air up, I contend that it helps get the cool air off the floor. Otherwise, there is a noticeable temperature gradient, and the cool air never gets to the thermostat, so the A/C runs too much. Anecdotally (is this a word?), I feel more comfortable with the fan sucking up in the summer. Am I just weird?

That's just fine for mixing the air, but does almost nothing for cooling your skin. And if you have the fan on high it'll do a pretty good job of mixing in either direction. Even blowing downward it is sucking the warm air from the ceiling and blowing it downward into the cool air below. With the warm air coming down the cool air then has nowhere to go but up.

If you have a good A/C and don't want to sit under the ceiling fan to stay cool you can have it run on low and upwards both in summer and winter. But a fan is more energy efficient than an A/C... as long as it's pointed at you so that it can actually have an effect. What annoys me is that a fan is only pointed at a small part of the room so as soon as you walk out of that area there's no point and you'll wish you set your AC lower instead.

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Re: Ceiling Fan Direction

Postby uncivlengr » Sat Oct 02, 2010 1:40 am UTC

Ignoring skin cooling effects, I don't think the fan blowing in either direction creates the same amount of circulation.

As was pointed out earlier, the fan draws air in from all directions on the inlet face, whereas the air exits with momentum that carries it in a more linear manner away from the fan.

If the fan is blowing upward, the air drawn into the fan is the air closest to it, which is at the ceiling. When that air is pushed through the fan, it hits the ceiling, disperses, and flows out to the walls. At this point it's not going to have a significant amount of momentum to carry it all the way to the floor, so it's going to then be drawn back into the fan. This creates a circulation loop mostly contained in the upper half of the room.

Now, if we reverse the fan so it's blowing downward, the air is drawn from the celing, pulling the air from around the fan inward, where it is forced down - it doesn't have any obstructions, so it's going to make it further down to the floor before losing momentum. This creates a much larger loop, circulating a larger portion of the air.
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