Point fans outward or inward from windows?

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thelehmanlip
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Point fans outward or inward from windows?

Postby thelehmanlip » Tue Jul 13, 2010 3:36 pm UTC

Assuming the window is large, (at least twice the size of the fan), and in a closed room with no doors open (but perhaps other windows), and that it's cooler outside than in, I theorize that blowing air OUT the window would cool down the room more. I base this because when you point a fan inwards, (the fan on the sill, half-way in and outside), there is a portion of the air being pushed by the fan that is air from inside the room. However, if you point it outwards, a very high percentage of air being moved is coming from inside, being pushed outside. This causes a pressure difference, making air come in through other portions of the window, or other windows in the room, being more effective at cooling down the room.

Any credence to this idea?

sikyon
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Re: Point fans outward or inward from windows?

Postby sikyon » Tue Jul 13, 2010 3:48 pm UTC

Yeah it will make your room cooler to blow air out the window, because that will draw more cool air into your house from the other windows.

If that is the only window in the room I expect it to not work though - cool air is just comming in from that window and being cycled back out.

In practice blowing in is usually a better idea (at least in my appartment). The fridge generates alot of heat and the kitchen does not have any windows, so I need to cycle air through the kitchen to reduce the heat better.

What would be best, though, is to have your fan outside blowing air inwards (this is my current setup, on my patio).

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Re: Point fans outward or inward from windows?

Postby thoughtfully » Tue Jul 13, 2010 3:57 pm UTC

Whichever way the fan is blowing, the amount of air moved will be the same for a given speed. Averaged over the long run, this is all that matters. In the short term, you will get more cold outside air piling up near the fan if it's blowing in, which might be noticeable if the temp difference is large and the circulation inside the room is poor.

If the fan is blowing on you, you feel the cooling effect of the moving air over your skin. This doesn't work symmetrically, because the air moving over you will be cooler and moving faster than if you were in the same position with the direction reversed.
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Re: Point fans outward or inward from windows?

Postby Coffee » Tue Jul 13, 2010 4:10 pm UTC

Sounds like a particularily easy experiment to set up; why not run some tests and let us know?
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Re: Point fans outward or inward from windows?

Postby skeptical scientist » Tue Jul 13, 2010 5:04 pm UTC

If you have a single window to work with, blowing the air inward is better. Obviously this generates more air movement inside, which helps to make the air cooler. Also, note that with only a single window, blowing air in results in a higher pressure than blowing air out, since the rate at which air moves in/out to equalize the pressure isn't as great as the rate at which air moves in/out when pushed by the fan until some pressure difference builds up. So the amount of air moved by the fan will exceed the amount of air moved due to pressure differences. Since the goal is to move cool air in, rather than pushing hot air out, pointing the fan in is more effective.

If you have windows on opposite sides, however, the best thing to do is to point one fan in and one fan out as that will give you an excellent breeze that also moves a lot of inside air out and outside air in.
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thelehmanlip
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Re: Point fans outward or inward from windows?

Postby thelehmanlip » Tue Jul 13, 2010 5:18 pm UTC

Coffee wrote:Sounds like a particularily easy experiment to set up; why not run some tests and let us know?


I actually did an science fair experiment in high school where i was changing the temperature in the room up and down multiple times. I cranked the heat up, and then when i needed it to cool down, it seemed (i.e., no empirical data to back it up) to be faster if I pointed the fan outwards. This is kind of where this comes from. If I could, I'd test it now and put some measurements to it.

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Re: Point fans outward or inward from windows?

Postby sikyon » Tue Jul 13, 2010 6:57 pm UTC

thoughtfully wrote:Whichever way the fan is blowing, the amount of air moved will be the same for a given speed. Averaged over the long run, this is all that matters. In the short term, you will get more cold outside air piling up near the fan if it's blowing in, which might be noticeable if the temp difference is large and the circulation inside the room is poor.


That's not quite true. Over the long run, the amount of air moved is definatly not all that matters. It does infact matter how much cold air you can draw into the room and how much hot air you can move out of it. Remember that the fluid profile of the wind generated by a fan is not symetrical. The rear of the fan will suck air in from all directions and the front of the fan will blow air out in only one direciton, over a smaller cross section.

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Re: Point fans outward or inward from windows?

Postby fooliam » Tue Jul 13, 2010 11:33 pm UTC

What about the air sucking in the surrounding air? If the fan is blowing cool air in...the some of the warm air being pushed out the window would be caught back up and pushed back in correct? And if the fan is blowing warm air out, would some cold air be caught and blown out along with the warm air? If so, I would assume that, provided the fan is in contact with the bottom surface of the window sill but not the top surface, that pointing the fan inward would result in slightly more cold air entering the room. I think this because, were the fan pointing outward, cold air passing over the top of the fan would fall into the current of air being sucked in by the fan (colder=denser=sinking air), reduing the amount of cold air entering the room. With the fan pointing inwards, the warm air wouldn't have any space to rise from below the fan and re-enter the air current.
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Re: Point fans outward or inward from windows?

Postby Xanthir » Wed Jul 14, 2010 1:01 am UTC

If you have a single window, yes, partially. If you have multiple windows, then not nearly as much, as it's much easier for the hotter air to escape out the other windows.
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achan1058
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Re: Point fans outward or inward from windows?

Postby achan1058 » Wed Jul 14, 2010 1:28 am UTC

There's one issue that you haven't considered yet. What type of fan are you using? If it is one of those fans with a large rear sticking out, blowing out might be better (with more than 1 window), since you can actually hug the fan to the window, where you cannot do that blowing in.

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Point fans outward or inward? An experiment

Postby DeVoto » Tue Sep 03, 2013 10:20 pm UTC

This is quite an old post, but I've got a similar problem and I'm curious for the best option.

My situation is similar to thelehmanlip(op). I've got one large window for ventilation and a small fan which can fit between the screen and the fairly heavy blinds with three variable speeds. My house does have central AC, but the AC vent, which is located beneath the window, is partway beneath the bed. It works well, but only if I have my door open and my fan pushing air out, other wise, it hardly has an effect.

My biggest problem is this: I'm 20 years old living at home and want my door closed for privacy, but when my door is closed the room's ventilation appears to drop to zero and as certain family members prefer to have the AC off during the night, my desire to have a cold room when I go to bed, is very difficult to achieve. So I'm here to do some testing and see if I can come up with some information for anyone interested and more importantly myself.

For my best guess, I'm going along the similar lines of skeptical scientist. That blowing air in will increase inside pressure and force the unwanted, hotter air out (though I'm not sure negative pressure can happen with just one window).

I've got a few questions before preceding. First, does position the fan on the top half on the window, as apposed to the bottom half, have an effect on cooling the room down? Or is it simply the same? Second, does a full open window with the fan taking up a quarter of the window effect the cooling of the room? And lastly does pushing air in vs out have an effect on the cooling with these two options? From these options I'm guessing having a full open window with the fan at the top pointing out (pushing hot air from the top out while drawing cold air in below the fan).

So after payday I'll get to running a few different trials during the next week or so. I'll be sure to update you guys once I've started.

-DeVoto


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