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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?

#81

Post by colanderman »

kenhwang wrote: post #28
cbanek wrote: post #9

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 fo…

Just to add, for computer cases, you want it to be a high pressure zone because intake fans generally have filters in front to catch the dust before it gets into the case. Negative pressure means dust gets sucked in from the cracks.

Same applies for house fans -- you want the intake fan to be slightly stronger than the exhaust fan. Else the negative pressure will draw outside air in through cracks and all the fun smells that come from those places.

(I have a combination energy recovery ventilator/HEPA filter that has has a restriction filter on its exhaust for this reason. Else it would just suck crappy air in from the basement instead of fresh air through the filter.)

If you have to choose one, choose an intake fan for this reason. Best if you can leave another window slightly ajar.

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

#82

Post by jschwartzi »

Sammi wrote: post #42
amelius wrote: post #35

Earlier quoted context omitted.

Better yet: keep yourself wet while using the fan. The evaporation of water molecules cools you down.

I've walked around music festivals for days in the sun with a water bottle, and used more water for soaking the hair on my head than for drinking. Works wonders :)

When I'm in a snowfield under the sun I'll smear snow on myself and stuff it down my shirt. Has the same effect.

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

#83

Post by colanderman »

cbanek wrote: post #9

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 fo…

I strongly disagree on the setup with only one fan. Having only an exhaust fan in the room will draw nasty air in through cracks in the house's structure, possibly stuff with e.g. mold spores. My wife could never remember which way was which for our window fan; I could tell by smell when she had left it exhausting for more than an hour.

If you have only one, set it to intake and leave another window open a bit (maybe like half the fan size). You can confirm that air is exiting that way by hanging some strips of tissue paper in it.

Positive pressure (blowing in, from outdoors) is also the right thing to do if you are trying to keep smells from other rooms at bay. Whereas negative pressure (blowing out, to outdoors) is best to keep a smell confined to one room (e.g. this is how bathroom fans work).

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

#84

Post by cgriswald »

taneq wrote: post #18

If this is because you want to feel cooler, place the fan so it blows cool air inwards onto you . You'll get far more benefit from having air blown on you than you will from just dropping the ambient temperature around you.

Just be aware that you will also be getting a larger dose of dust and allergens.

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

#85

Post by masklinn »

raldi wrote: post #80
Cerium wrote: post #79

Earlier quoted context omitted.

Yes, fans usually have a specification on the pressure they can create in inches of water or millimeters of water. I think it is because you can measure it yourself with just a tube and water- open end manometer.

How?

Put bottom end of tube in water, put fan at top of tube drawing out, turn fan on, see how much the water climbs inside the tube?

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

#86

Post by masklinn »

lathiat wrote: post #60
tomohawk wrote: post #43

Earlier quoted context omitted.

If the goal is to make the room cooler, your teacher is right. If the goal is to make the students cooler, then the 4th graders are right. 4th graders sweat, and the fan will cause convective cooling of the 4th graders.

This is a concept often poorly understood in basic cooling design. Even without sweat, if your AC vent is blowing directly on you the temperature it is at needs to be vastly different than that of air being using to more actively cool the entire room. This is often a key difference between Car and Room AC systems. There’s also a surprising gradient based on how consistent the temperature is top to bottom (and you get…

> This is a concept often poorly understood in basic cooling design.

Really? You'd think windchill is a well-understood concept.

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

#87

Post by m0rose »

colanderman wrote: post #83
cbanek wrote: post #9

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 fo…

I strongly disagree on the setup with only one fan. Having only an exhaust fan in the room will draw nasty air in through cracks in the house's structure, possibly stuff with e.g. mold spores. My wife could never remember which way was which for our window fan; I could tell by smell when she had left it exhausting for more than an hour. If you have only one, set it to intake and leave another window open a bit (maybe…

Agreed. In my own experience, it's much more effective when cooling a room to pull the cool air in than to try to exhaust the hot air from the entire house so that cool air will eventually trickle into the room you're trying to cool. Changing from pushing the air out to pulling it in directly was an epiphany for me when I was younger and lead to many nights of better sleep.

Even better, now that I own my own house, is a high powered fan in one window pushing the air out and another (lower-powered) fan in a different room pulling the air in with an additional window open. You can stand in front of the open window and feel the additional cool air streaming in. It's a beautiful thing.

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

#88

Post by cortic »

cbanek wrote: post #9

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 fo…

You never want matching airflow in a PC case, it creates pockets of heat, frying components, usually the top disc drive or sometimes the PSU.

Positive pressure is very good at assisting the PSU fan and any radiator exit fans you have, negative pressure is very good at cooling the edges of the MoBo and upper drives in a tower system. Negative is generally cheaper, so its usually negative.

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

#89

Post by neltnerb »

cbanek wrote: post #9

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 fo…

Height is also important if you've got a multi-floor house. In upper stories, make the fans face outward and on the ground floor make the fans face inward and you'll set up a continuous air path throughout the whole house favoring cooler air.

Personally for a single fan in a smaller room where you're near the window, I'd say just point it in. That way you get fresh air right where you are sitting, you get the breeze, etc. In the other direction you won't get the cooling effect on your body from the breeze.

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

#90

Post by neltnerb »

realpeopleio wrote: post #77
realpeopleio wrote: post #37

My 4rd grade teacher was convinced of the science of using two fans to cool off the classroom: one blowing cool air into the classroom from the hallway and one fan blowing hot air out of the classroom through a window. The kids that sat near the fan blowing in the room from the hallway were quite content, but the kids by the window were always complaining and they would secretly turn the fan in the window around so i…

I should add that my teacher would think we couldn't understand the science or concept behind airflow going through the classroom to cool us off. He'd explain it over and over and get frustrated that so many students didn't seem to "get it." But really they did get it, it just wasn't working for them. I wonder if the arguments about scientific things nowadays aren't similar. It may not be that someone is stupid or do…

Sounds like a perfect opportunity for the teacher to demonstrate the scientific method by setting up thermometers throughout the room and recording them throughout the day.
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