
Ask HN: Do I place the fan so that it blows air inwards or outwards? - optimmal
I am positive that there&#x27;s a huge difference between the theoretical model and reality here.<p>Let&#x27;s assume it&#x27;s 30 celsius inside the room and &lt;20 celsius outside.<p>Enjoy your Monday my thermodynamics friends!
======
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.

~~~
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](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](http://www.purifresh.com/cleanroom.html)
They can be linked (as
[http://www.purifresh.com/freshairsystem.html](http://www.purifresh.com/freshairsystem.html))
or used separately. (Apparently this is out-of-stock and being redesigned.)

------
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.

------
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](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](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.

------
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.

~~~
misnome
This isn’t exactly wrong, but ignores the fact that the primary purpose of a
fan in a darkroom isn’t to clear dust, but extract chemical fumes. You
probably want fumes pumped directly outside, not back through the gaps in the
door into the general building.

------
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.

~~~
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.

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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.

------
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.

------
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.

~~~
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.

------
LocalMan
To cool summer bedrooms, blow the air out the kitchen window, and open the
bedroom window. This will draw cool night air in the bedroom window. You may
be sweating at 7 PM but you might need a blanket at 1 AM. This works
brilliantly. And putting the fan out the kitchen window means the bedroom is
quieter.

To heat winter rooms that have an underpowered radiator, blow the fan onto the
radiator. The job of heating the room is the same as cooling the radiator.
This works very well but the room downstream (on the steam line) will be
getting less heat. In practice, that was the neighbor's apartment. The
freezing neighbors complained to the landlord who eventually tracked it down
to my place. Vigorous discussions ensued.

I also discovered that a HEPA filtration unit, even sitting on the floor,
helped my allergies a lot. It was not cheap, but when I find my nose running
again, it means that something had turned off the filter. For years I had
thought that HEPA was a scam. Not so.

------
analog31
Are you trying to cool yourself, cool a room, or cool the whole house? Can you
experiment and find out what works for you?

Cooling the entire house is an expensive proposition if you're just trying to
get to sleep in one room.

------
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

------
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.

------
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.

------
phillipwills
We have a 2 story house where the bottom story is a daylight basement. When it
gets hot, it usually stays pretty cool downstairs, so we hang down there
during the day. In the evening, as soon as it's cooler outside than upstairs
inside, we open the back door, both downstairs and upstairs, and turn on a
high powered fan blowing air up the stairs... It is really effective at
cooling the house.

------
darkstar999
I'm convinced that outwards is the most effective in this scenario:

Open your bedroom windows on a cool spring/summer night. On the opposite end
of the house, put a fan in a window blowing outwards. This cools the entire
house, giving your bedroom the freshest air. In the morning, close all the
windows. This leaves the house cooled and drastically reduces your A/C usage.

Blowing inward doesn't have the same effect of pulling air through the whole
house.

------
innocentoldguy
I usually place the fan so it faces outside. I figure out which way the wind
is blowing, open two windows on opposite ends of the room/house that
correspond with the wind direction, and then place the fan, facing out, on the
downwind end of the room/house. My thinking is that the wind will push itself
into the house through the fan-less open window, but that it will then lose
momentum. Then, the fan on the downwind side of the house will help pull the
air back out of the house, creating circulation that doesn't have to fight
against the wind.

Recently, I bought a whole-house attic fan. Now, I open a window on the upwind
side of the house and turn on the attic fan, which is enough to suck a toupee
off someone's head. The attic fan both circulates cooler air and also clears
out the hot air built up in the attic, which helps a lot.

------
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.

------
thermodynthrway
The direction doesn't matter nearly as much as blowing the fan through a
large-ish opening.

You want the air from the fan to accelerate other air on the way out/in. This
is the same principle as turbofan engines, in which a higher bypass ratio
(bigger fan) attached to the same fuel burning jet core is more efficient.

Why? Because the relationship between velocity and volume is not linear. Takes
more energy to move the same amount of air the faster you accelerate it.

So ideally you want to position the fan in a large opening far enough back
that the fast moving air can get a bunch of other air moving as well. Trade
the high velocity fan air for a much higher volume.

------
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.

------
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..

------
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

~~~
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.)

------
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.

------
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.

------
karmakaze
Use all available upper exhaust vents.

I'm not the top floor in the corner room which is the hottest room. The entire
unit is per floor is much warmer than any other level. The best non-AC cooling
I've been able to achieve is to run the bathroom fans with the bathroom
windows closed. It pushes out warm air near the ceiling and draws in cooler
air from windows and lower parts.

------
g105b
Too many assumptions to answer precisely. Chances are you'll feel cooler if
the fan is blowing inwards because that will help your body reduce its
temperature via evaporation of perspiration.

There are much more complex cooling setups that can be achieved that utilise
outward-blowing fans though, but a single fan can't will cool _you_ faster if
it is blowing onto you.

------
amelius
Here is a DIY project for an air-conditioner, using a fan:

[http://www.thegoodsurvivalist.com/youll-be-incredibly-
surpri...](http://www.thegoodsurvivalist.com/youll-be-incredibly-surprised-at-
how-cool-this-diy-poor-mans-air-conditioner-will-keep-your-car-camper-tent-or-
room-step-by-step-instructions-and-proof-that-it-works/)

~~~
finnthehuman
The usefulness of a swarp cooler depends heavily on ambient humidity.

------
innocentoldguy
In my computer setups, I place three intake fans each on both the front and
bottom of the case and then place two exhaust fans each on the top and back.
Again (see my other post), the idea is to maintain the natural airflow. Since
heat rises, I want my exhaust fans to assist with that natural flow rather
than fight against it.

------
shmerl
For computer cases, take in account dust. You can use dust filters on the
intake, and create positive pressure in the case. So you'd need more air
blowing in, and less blowing out in order for dust not to accumulate inside
(with negative pressure, dust will start actively sneaking in through holes in
the case).

------
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.

------
reitanqild
Another design input:

If there's lots of humidity inside the room you'd probably prefer negative
pressure in the room.

Reason: positive pressure can pressure moist air through walls and ceilings
and when it hits the cold air on the other side of the insulation it condenses
in thr construction and causes rot (or corrosion).

------
iamgopal
In to room, via nice HEPA filter, for cleanliness.

Ignoring that, In to the room. Fan can not create much vacuum but can create
much pressure.

Let me give you my most handy example. Imagine a queue. A push can cascade
from last person to first person, but a pull can only be apply to last person.

Hence, pressurise the cabin.

------
sdcooke
The datagenetics blog has a pretty deep dive into this question!
[http://datagenetics.com/blog/july32015/index.html](http://datagenetics.com/blog/july32015/index.html)

------
elorant
While we're on the subject, one thing that I never understood is why fans are
directly on top of the heatsink instead of floating a centimeter above it,
allowing the air to flow more easily, and also make less noise.

------
jimnotgym
Can I pose a similar question? I have a cieling fan in my conservatory that
can blow up or down. Which way is better? It is pretty close to the cieling if
that matters.

~~~
gargravarr
When we fitted a ceiling fan in our living room, the instructions stated that
in summer, it was best to have it blowing upwards (sucking air up from floor
level) because that air would be cooler, and in winter from the same
principle, you could blow warmer air down onto the room occupants.

In practise it didn't work that well - the temperature difference in a couple
of metres of air space (in a single average-height room) is pretty minimal. We
left the fan blowing downwards to use the windchill effect in summer - that
worked well enough.

------
ilogik
I'm surprised nobody posted this:

[https://www.youtube.com/watch?v=QjYli6itP38](https://www.youtube.com/watch?v=QjYli6itP38)

~~~
znpy
Also the follow-up:
[https://www.youtube.com/watch?v=dLX54ounENY](https://www.youtube.com/watch?v=dLX54ounENY)
\-- 1 Year Airflow Experiment - The Stunning Conclusion

------
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.

------
flatfilefan
If it’s placed under the ceiling - to blow outward because warmer air rises up
and would be leaving the room through that opening if there were no fan.

------
FPGAhacker
Sounds like a question for Randall Munroe.

------
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/](http://n-gate.com/hackernews/)

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

------
cleanyourroom
Out.

------
jimjimjim
is there a breeze? if air is moving either in or out unaided then the answer
is to assist that air flow.

------
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://superuser.com/questions/192920/should-case-fans-blow-air-
into-or-out-of-the-case)

[https://physics.stackexchange.com/questions/71620/on-a-
hot-d...](https://physics.stackexchange.com/questions/71620/on-a-hot-day-when-
its-cooler-outside-than-in-is-it-better-to-put-a-fan-in-an)

------
hellofunk
Depends on your goal.

Do you want to:

1) Cool down your room?

2) Warm up the outdoors?

3) Warm your room further?

~~~
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.

------
smilesnd
I am a man don't need a fan.

------
stu3
Most replies intelligently include open windows. Running an electric fan in a
closed room with unopened or no windows may prove fatal and can lead to
irreversible Fan Death. It's not a good way to go.

------
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.

