Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

news.ycombinator.com

Post reply on HN
Print view View original

51–60 of 120 posts

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

#51

Post by lmilcin »

Very technically, it is better to have the fan IN the room blowing OUTSIDE through a large opening. The air accelerated by the fan accelerates more air and that air goes outside. It mixes with the outside air so even if some of it goes back to the room it will already be cooler.

If you have fan in the window blowing into the room it has less potential to multiply the air it takes, it will typically be also already IN the room itself which will greatly decrease its efficiency.

On the other hand if your goal is to cool yourself when sitting at the des it is generally better to place your fan blowing in, this means you can direct the flow of air to wherever you want. This means you can direct it at your desk to cool yourself by just the air movement alone. When you place your fan blowing outside you loose that ability. Blowing the air at you means you can cool yourself even if the air temperature outside is higher than inside!

If your goal is to move as much air through your room then you need to take into consideration the following:

- do you have other openings in the room (like doors... huh)? - can you have flow of air through the building -- typically there will be pressure difference between two sides of building unless the air is completely still

If you can have your door and the window open, observe the direction of air already naturally flowing. Then put your fan so that it blows with already moving air, for maximum efficiency.

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

#52

Post by kosma »

There is a far bigger problem than thermodynamics in play, namely: dust. Not just in computer cases: it also applies to places like photography darkrooms. The usual pattern of placing a single blowing-out fan in a ventilation duct is all wrong: it pulls air in from the cracks in doors and windows. The dust-proof way is to make the fan pump the air into the room, but also install a HEPA filter on the air intake pipe/fan.

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

#55

Post by INTPenis »

So far I've managed without fans because my home has one side in the shade and one side in the sun so I just create a draft and pull all the blindes down to avoid direct sunlight.

Being a Swedish summer, maybe around july august I'll need to take the fan out of storage and then it's just to get a draft on myself. Not to move any air around. The draft between the two sides of my home does that better.

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

#56

Post by sureaboutthis »

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…

As one who designed medical equipment (though not the mechanical engineer) this is the answer.

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

#57

Post by mrhappyunhappy »

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…

Speaking of school, and forgive me for not adding anything to discussion at hand, my biggest distraction was always sweat. I’d get so hot and sweaty most of the times that my butt would leave a sweat stain on the seat. I’d say I spent a good portion of total learning time just worrying about the embarrassment that would result from having a sweaty butt and leaving a stain. Mind you I was not overweight, just a very sweaty kid. I don’t know this for certain but if my school had ac I may have actually learned better by paying attention in class instead of worrying about my butt. Educators take notice.

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

#58

Post by namibj »

adrianN wrote: post #6

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…

Can you recommend any specific software? I'd finally have a reason for the 3d-scanner in my phone... (getting the model at a suitable resolution of <3cm voxel-cube size to not have to model it all from hand)

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

#59

Post by cbr »

strawcomb wrote: post #30
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.

Fans -> cool human via wind-chill [1] Air conditioning -> cool human by cooling environment [1] https://en.wikipedia.org/wiki/Wind_chill

You can also use fans to cool the environment, trading hot inside air for cooler outside air. For example https://en.wikipedia.org/wiki/Whole-house_fan

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

#60

Post by lathiat »

tomohawk wrote: post #43
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…

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 into trouble with this on reverse cycle AC systems)

And lastly it’s easy to ignore radiant heat from both walls/ceilings as well as even large monitors.

It all plays in together.

Post reply on HN