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

190 points by optimmal ↗ HN
I am positive that there's a huge difference between the theoretical model and reality here.

Let's assume it's 30 celsius inside the room and <20 celsius outside.

Enjoy your Monday my thermodynamics friends!

119 comments

[ 2.9 ms ] story [ 158 ms ] thread
hellofunk
Depends on your goal.

Do you want to:

1) Cool down your room?

2) Warm up the outdoors?

3) Warm your room further?

[deleted]
(comment deleted)
adtac
Cooling down your room and warming up the outdoors is the same thing, if it's a purely mechanical problem, because the energy to warm up the outdoors has to come from somewhere and the room is the only other entity.
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.
mburst
Fans don't really cool, they just move air around. Thus you'd want to blow the 30 degree air toward the 20 degree air to decrease the temp inside the room.
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.
perl4ever
I agree on "maximally far apart", but more in the horizontal than the vertical direction.

Extract hot air in one location, preferably up high, and let cold air in with a window that is as far away as possible - it doesn't really matter high or low.

roel_v
"You should probably do some computational fluid dynamics on a model of your place"

Oh great, there goes my Monday. Thanks buddy.

pmoriarty
Or you could just experiment, try both ways, and measure the temperature differences.
FraKtus
Exactly what I do, use a standard temperature sensor, load your system, make the CPU and GPU cry :-) and see how it goes with cooling the inside of the system.
namibj
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)
bkanber
Overkill for the hobbyist, but ANSYS products like Fluent and CFX are what the professionals use. Quite pricey though.
namibj
Yeah, if I had the money for such software, I wouldn't need to ask. I'll assume you don't know any affordable substitute.
l1n
OpenFOAM might work for this purpose.
ss64
If you look at the design of windows on old houses from about 1700 to 1940 they facilitate exactly this. They open at the top and/or bottom to allow the circulation of air. Hot air will naturally rise and exit from the high up window. When air conditioning took off in the 1950s this became less important.
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.

Moru
Assumes small room with only one window and can't open anything else: Place the fan in the lower part of the window blowing down inwards. Sucks in air in bottom half, warm air goes out above the fan.
veli_joza
You are answering a different question. To put it in your terms, do you think your solution is more efficient than putting fan in upper part sucking the air out, and what's your reasoning?
Moru
Well, it's more efficient to place something on the windowsill than to hang a rotating fan from the upper part of the window, that will just move around and most likely blow downwards, obstructing the airflow.
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)

kenhwang
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.
jjcm
Also while extremely trivial here, higher pressure air has a higher capacity to transfer heat.
lmilcin
While this is technically true, the difference is very minor at the typical pressure ranges being used.

The highest performing PC fans create about 10mmH2O pressure while typically you can expect about 1mmH2O pressure compared to 10mH2O of the atmosphere. This translates to densening of the atmosphere around components by 0.1% or 0.01%, respectively.

Correct placement and design is by orders of magnitude more important than the increased efficiency due to higher density of air.

You could see much more effect by making your air more humid (humid air has higher heat capacity), which will be also much more pronuncable than the increased density due to pressure.

mberger
TIL about mmH2O. Is this common in HVAC? I thought pressure was mostly in mmHg or bars(millibars).
Cerium
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.

raldi
How?
masklinn
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?
[dead] lightedman
"The highest performing PC fans create about 10mmH2O pressure"

I'm guessing you've never worked with a Delta fan. Those things produce enough pressure to literally suspend themselves in mid-air.

phkahler
Thanks for mentioning filters. I'm just in the process of designing an ITX case specific to a board/CPU and plan to partition it so the CPU fan is also the case fan. I hadn't thought about dust. Good point.
FullyFunctional
Dryer sheets :) I don't remember where I found this tip, but recycling used dryer sheets as air filters works fairly well (definitely beat having nothing).
mark212
Or use the new ones for a delightful fresh scent!

Kidding!

colanderman
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.

toomuchgov
What is the make/model of your ERV?
colanderman
It's a window unit: http://www.purifresh.com/erv.html It's the only such kind I could find when we were in the market a few years ago. (Was renting a place with mold issues.) Works well for a single room. It's wonderful to pull fresh air in during the winter.

We also have its accompanying HEPA unit: http://www.purifresh.com/cleanroom.html They can be linked (as http://www.purifresh.com/freshairsystem.html) or used separately. (Apparently this is out-of-stock and being redesigned.)

saalweachter
My vague understanding is that negative pressure in a house is also how you get radon gas.
sureaboutthis
As one who designed medical equipment (though not the mechanical engineer) this is the answer.
colanderman
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).

m0rose
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.

spdustin
In some zones, whole-house ventilation fans (which are installed in the ceiling below the attic) are excellent and efficient ways to cool and/or ventilate a house quickly—by exhausting the air in the living area of the house up and out through the passive vents in the attic. Open some windows, turn on the whole-house fan, and the air in the house is fully exchanged with fresh air from outside very quickly.

It helps to reduce stale odors, bring in fresh air, reduce indoor allergens...also handy when you accidentally burn something in the oven. :)

mark212
I think the key there is to open the windows at a time when it’s cooler outside. Without sufficient openings to pull air in, the pressure differential is inefficient
cortic
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.

neltnerb
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.

Steel_Phoenix
I also like to take prevailing wind into account. Even if it's a small breeze, fighting it can really drop effectiveness.
pbhjpbhj
And consider shadow - let in colder air from shaded areas rather than warm air heated by surfaces in direct sunlight.
jimjimjim
is there a breeze? if air is moving either in or out unaided then the answer is to assist that air flow.
delinka
Let's further assume that I'd rather have the room cooler than 30C. Also, regardless of which direction the fan will blow, there also needs to be another place for air to move into and out of the room. So let's also assume that the window is the only opening, but the fan doesn't fill the opening produced by raising the window. One more assumption: I can point the fan with freedom in most any direction once it's placed.

I believe I'd pull the cooler air into the room by having the air blow in. I might also point the fan up and away from the other side of the window (up because I want to disturb/displace the warm air higher in the room; away from the non-fan side because I don't want my cool air to be pushed right back out.)

I don't want to cool the warmer air, I want to replace it with cooler air.

perl4ever
When I had an apartment with several rooms and non-working air conditioning, what I did was to open a window near the front of the apartment, in the dining room, and set up a large fan to blow out the window. Then I would open a window in my bedroom, at the opposite end of the apartment, to provide inflow. This was very effective at reducing the summer inside temperature to that of the outside at night, where the purported "heat pump" built in did essentially nothing.

The only issue was that it also sucked rain and pollen in as well.

[dead] nimchimpsky
Using a fan actually increases the temperature of the room due to the motor providing heat.

Get some form of refrigeration, air conditioning.

If we are talking about computer cases, out.

dredmorbius
Fans achieve two effects: ventilation (air exchange between two voids) and circulation (air distribution within a void). The first will achieve the goal of replacing a fluid volume with another (hotter/colder), but may not evenly mix the fluid within the conditioned volume.

Overpressures tend to prduce airstreamss or turbulence downstream, but underpressures don't induce much flow beyond the immediate orifice. Unless convection, ducting and venting can reliably create flow patterns, blowing air in to a conditioned space is a better option thansucking ot out, though if you want to control for emissions (say, filtering exhaust), you might want to combile both.

There's a classic physics experiment showing what happens when you run a water sprinkler, the kind with a spinning arm and one or more jets, backwards (sucking rather than blowing), and hhow it doesn't spin. Feynman discusses this in a story.

The fan(s) themselves contribut some additional heat to the space, but this is fairly negligible, especially relative to circulation effects.

If the heat source is external (e.g., incident sunlight), then natural airflow patterns which might draw air through a hotter chamber (say, an attic or other roof void) could have the opposit of the intended effect. It's key to check overall circulatory flow.

And if internal systems, or people, have significant evaaporative or similar cooling mechanisms, increased internal circulation will greatly improve cooling capacity.

Forced inflow may increase dust or other factors, if not filtered.

TL;DR: If you've only one option, I'd blow in rather than suck out. You might want to consider overall airflow, circulation, and heat distribution or souces (and sinks) within the void. Blowing onto rather than away from the primary heat source (in a cooling scenario) is likely best. If heat sources are external, exhaust from the top of the void is optimal.

Intakes should generally be low, exhausts high (for coolng).

For a large enough and complex enough (multiple chambers, zone, multiple heat sources, significant convection effects, dead zones), you would likely want a combination of intake, circulation, and exhaust fans, though I'd prioritise them in that order.

Disclaimer: I'm a space alien cat, not a fluid thermodynamics engineer.

This questtion has also appeared elsewhere:

https://superuser.com/questions/192920/should-case-fans-blow...

https://physics.stackexchange.com/questions/71620/on-a-hot-d...

taneq
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.
strawcomb
Fans -> cool human via wind-chill [1]

Air conditioning -> cool human by cooling environment

[1] https://en.wikipedia.org/wiki/Wind_chill

cbr
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
innocentoldguy
Whole house fans are great. They cool the house, clear out the 150º air radiating from the attic, and also take a lot less energy than air-conditioners. I use both, but my air-conditioner runs a lot less since I installed the attic fan.
taneq
Same. Well, it's an evaporative air conditioning unit but 90% of the time we don't turn on the evap function (which is only really useful on days which are very dry and not too hot) but just run it as a fan in the evenings. Blows all the hot air out of the house and replaces it with cool night air in minutes.
croon
Assuming temperature in room is lower than your temperature (likely).

It only accelerates the speed at which your skin temperature moves to match the air temperature through convection.

If you can move air elsewhere to lower the temperature of the room, that is better long term.

jakobegger
The cooling effect of moving air is mainly because of evaporation of sweat. So it is more important how humid the air is, than how warm. Warm dry air blowing on you will cool you, warm humid air won’t.
amelius
Better yet: keep yourself wet while using the fan. The evaporation of water molecules cools you down.
Sammi
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 :)
jschwartzi
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.
cgriswald
Just be aware that you will also be getting a larger dose of dust and allergens.
vortico
If you blow air in, you're sucking bugs in and blowing air out of your central air filters. If you blow air out the window, fewer bugs. That's enough to make me not care about the thermodynamic answer.
dgoldstein0
This assumes there are enough bugs to be a problem. Some places this isn't the case. Also more likely to have bugs at lower floors of buildings as they tend to hang around trees and plants.
lstodd
I'd worry more about dust. In a big city, just leaving windows quarter-opened for a week results in everything covered with a visible layer of dust and finely-ground rubber
adrianN
Maybe you should move to a country with stricter air quality laws. I leave my windows open all summer and I can't observe any ground rubber in my apartment.
majewsky
Assuming you're either near a big road, or your city has a major problem with air pollution. I'm near a sort-of important road in a German city and I don't observe anything of this magnitude even though my windows are fully opened quite regularly. (Air pollution is not that bad in my city. Many other German cities are considering driving bans.)
marcosscriven
I like to think of it like emptying a bottle - if there's only a small hole it goes glug-glug, as occasionally air forces its way into the bottle.

It's the same with air - there has to be a good way to get cold air in and hot air out with minimal turbulence from the two flows.

I think you're better off blowing cold air in (there's no such thing really as 'sucking' in physics), as attempting to blow hot air out doesn't necessarily mean it's going to be replaced with cold air from the outside, rather than warmer air elsewhere in the building.

lstodd
Fans are more efficient at pulling air out than pushing air in. So if you don't want to waste energy put all the obstacles - filters, rooms, etc - on the 'in' side and keep the exhaust as clear as possible
Rapzid
In your scenario, inward. You are looking to achieve the windchill effect. Maximizing this with evaporative cooling would be ideal. You'll need a fluid cooling system that seeps liquid to the surface for this..
anujsharmax
Everyone seems to be assuming that you have a big fan which actually changes the air flow in the whole room in a major way. If that's the case, great, put it any way you like - it doesn't matter.

For people like me, who use a cheap fan in the summer. Put it in front of you facing towards you. You will feel cooler.

If you are still interested in CFD I can give you a hand, but this is answer we are going to get from it anyway.

cimmanom
Fans big enough to significantly change air flow can be cheap. Window size box fans were $20 at the drug store last I checked. Build quality can't be great, but I had a fan like that last 5 years or so through heavy use.
z3ugma
I agree with you - it's similar to the principle of a space heater in the winter. You point it at yourself and heat only a small zone. In the summer, point the fan at yourself and use evaporative cooling to cool just a small zone.
palad1n
Growing up, we had no air conditioning. Instead, we had 2 fans upstairs, blowing out two different windows. In this way, 3 rooms all got air sucked in the windows. (We shut the downstairs windows, since no one slept there.)
segmondy
Actually the way to do it is to have the upstairs fans blowing out the windows, then the downstairs window open. This is more effective at night when the outside temperature has dropped and the house is still hot. Hot air rises, so you will get cool air pulled in. This is how whole house/attic fans work.
realpeopleio
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 it was also blowing in. This would infuriate our teacher when he would discover it and he would claim that the reason the classroom is hot is because the fans were no longer arranged to create air movement through the classroom. But the window kids weren't buying it.

So if sweaty 4th graders are any indication, have the fans blow on you regardless of whatever air movement is supposed to be made.

tomohawk
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.
lathiat
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.

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

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

lathiat
When it comes to basic price central ducted HVAC system installation you'd be surprised! At least in Australia.
pbhjpbhj
Also those by the window were maybe in direct sun.
latchkey
The issue is that the teacher needs to actually change the pressure in the room in order to create enough airflow. The fan in the hall needs to create a greater pressure (push air into the room) and the fan in the window needs to remove the pressure. If the fan in the window is stronger than the fan in the hall, then there will be a negative pressure (suction) to pull air into the room.

The problem is that the room probably has leaks such that if the fan in the hall is creating pressure, then the air doesn't all escape through the window. It'll escape through the path of least resistance or maybe even just spin inside the room (hot air rises too). If there is two doors for the room, some of it may be escaping under the other door. In that case, then air is just circulating back into the hallway. People also forget to take into consideration the expansion of hot air, which I've seen quotes for being up to 6x the volume. This complicates the pressures in the room.

As mentioned elsewhere in the discussion, the kids probably wanted to feel cool via evaporation since neither of the fans was moving nearly enough air to cool the entire room enough.

I run a couple large crypto mining farms in a pretty hot country. The 'kids' generate the heat and we have to move large amounts of air through them in order to keep them relatively cool. I've had to learn a lot about airflow design and it sounds very similar to what your teacher was trying to achieve.

mrhappyunhappy
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.
raldi
If all the other teachers were doing this too, the hall might have been negative-pressure enough that the air blowing in was circulating right back out before reaching the rest of the room.

That would mean the kids near the outlet fan were getting air that was just pulled in from outside via the window seams.

realpeopleio
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 doesn't get it, or even that they disagree, just that it's not working for them. The window kids understood that the room was supposed to be getting cooler, but it wasn't for them. They weren't against air flow, but it didn't seem to be working for them. Perhaps in theory the 2 fans in the classroom should have worked, but maybe the fans weren't strong enough, there were other places for the air to go (like the ceiling), etc. so that in practice it didn't actually work like the teacher claimed it would. Maybe there's a lesson there for public debates.
raldi
We used to make paper fans and the teachers insisted "they just make you hotter" (due to energy expended waving them) but it sure didn't feel like it.
neltnerb
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.
heavenlyblue
Sounds like a bunch of anti-vaxxers coming together and saying vaxxing doesn't work for them. This is the same reason why pilots are being trained not to listen to their own senses when flying: because those senses work against their reason.

I can see how a single fan blowing right into someone's face could be more efficient at cooling that person than the same fan working for the whole room; but frankly you also forget about all of the other pupils who aren't under the direct "sunlight".

smilesnd
I am a man don't need a fan.
jhanschoo
A bit of googling and a good dose of intuition leads me to think that the output area of a fan cools faster than the input. Reason being that air is more likely to be turbulent after a foil, and turbulent air transfers heat better.

On the other hand, if the purpose is to express, say, smell, blowing the smell out is the better solution.

fastbeef
I say this with love, but... Man, I cannot _wait_ until this guy[1] tears into this thread.

“Hacker News trips over themselves to invent HVAC basics from first principles”

[1] - http://n-gate.com/hackernews/

whatsstolat
Fun site. I had to open in incognito as it seems to block HN traffic.