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Ask HN: Do I place the fan so that it blows air inwards or outwards?

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Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#3

Post by acou_nPlusOne_t »

Well it depends on the heigth to floor - call that htf, in which direction the fan will work effeciently. Basically you have a flow of cold air inwards at ground level and a flow of hot air outwards at roof level. So the best solution would be to support the hot outflow, in the mid-layer, and let thermodynamics at the windows do the work.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#6

Post by adrianN »

I assume you want to move the cold air in and the warm air out. How to best achieve that completely depends on the shape of your room and the holes to the outside. You should probably do some computational fluid dynamics on a model of your place to see which blowing direction works better. It probably works best if you have separate inlets and outlets for the air that are maximally far apart, like on the ceiling and in the floor.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#7

Post by mosselman »

Assuming you want to cool down the room and there is 1 window:

Open the window in the room and open the door to the room. Ideally open a window somewhere else in the building and make sure there is a potential airflow from that other window to the one in your room.

Place the fan in the window of the room you want to cool down and let it blow outwards. Alternatively you can place the fan in the same way in that other window you opened.

Re: Ask HN: Do I place the fan so that it blows air inwards or outwards?

#9

Post by cbanek »

Like others have joked about, it's really about where you want the air and temperature to go.

Let's say you have three fans in a computer case, one at the back, one at the front, and one on top of the CPU. If they all blow out of the case, then you've basically created a negative air pressure zone, and you will be sucking air through the vent holes elsewhere, and through any gaps in the case. This may have effects for where the dust is taken in.

If all of them blow into the case from the outside, you've created basically a high pressure zone, and air will be taken from the room and pushed out through the same cracks and vent holes elsewhere.

A fan if it's pushing against high pressure will inherently push less air because it's pushing against air that isn't moving as well. A fan that is trying to pull air out can also become inefficient if it is working hard to suck air in to eventually push out. Therefore it's best to have matching airflow - what you push out, you should try to also pull in somewhere else in more or less an equal CFM (cubic feet / minute).

For a room, it's best to have one area that is your intake and the other side of the room be the exhaust. This way you aren't just exhausting air you just pulled in. If you just have one fan, I suggest blowing out, which will draw air in from other spaces, such as under the door, because of the negative air pressure by pushing air out.

(I've worked these problems with indoor gardens and HVAC systems, where there's a lot higher CFM fans and spaces, but the theory is basically the same)

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