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Discussion Starter · #1 ·
Recently, I've been seeing many people suggesting the following thing to people looking for help regarding the position of their fan in their case: "No matter what, put your top fans as exhaust because heat rises!"... Wrong! Something you must absolutely have for convection to be applicable is a totally passive environment. As soon as a fan is present, the environment is not passive and the effects of convection are pretty much irrelevant. Here are a few examples of where convection or top exhausts are relevant.

1) Passive PSUs

Some new PSUs are more and more present on the market, these are passive PSUs. These have no fans and therefore rely on passive air movement for cooling (also know as convection). Therefore, you should have the most vented part of it pointing UP (even if you have a bottom vent). This way, the heat that is coming from the PSU components will heat up the air and it will rise out of the PSU. Also, this will help cooling it because your GPU's fan should suck air from the PSU, helping in the cooling. It must noted that this will have almost no impact on the temperature of your GPU since passive PSUs are always very efficient and therefore produce a lot less heat than GPUs.

2) Strict air cooling

By strict air cooling, I mean no radiators. If all your components are cooled by air, you should normally have your top fan(s) as exhaust(s). But, this is not because of convection! This is simply because the component that is the nearest from the top fan(s) is the CPU, a major heat source. Therefore, you wan't the heat to be taken away from the heatsink as fast as possible. Also, this is a good way to keep directional airflow. The top fan will create a low pressure area near the CPU heatsink area which will cause the low pressure area to be filled by the closest source of air. That source should also normally be an intake fan providing cool air for the heatsink's fan(s). If you have a radiator on the back fan, they should always be set up as an intake for the best amount of air. Usually, they will be single radiators hooked up to a CPU cooler like the H Series from Corsair. Therefore, they are cooling the major heat producing component in that area. Because of that, the slightly heated air from the radiator will not cause the temperatures of any components to rise because there's nothing to cool. Of course, there's the VRM area that produces heat but it's a negligable source of heat and could probably be cooled passively in most cases (ie: anything but extreme overclocks on high TDP CPUs).

Now we have seen where your top fan(s) should be set up as exhausts. Sometimes though, you wan't them to be intakes and this is when most misleading advices come.

1) Watercooling

If you have a radiator (Like the H100 or most larger radiators that can only be installed on the tops of a chassis) on the top of your chassis or anywhere else for that matter, it should always be mounted as an intake. This is for various reasons. First, the radiator will only slightly increase the temperature of the air coming in which will not affect the temperature of other components by a significant margin. Also, the radiator is what's cooling the major components in terms of heat production (like a CPU or GPUs) and therefore, that slightly heated air intake will not affect important components except the VRM area which doesn't need much cooling in most cases anyway (ie: anything but extreme overclocks on high TDP CPUs).

Thanks for reading. If you have found any mistakes or believe I have forgotten something, I'd be glad to hear it from you.
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Quote:
Originally Posted by Snowmen View Post

2) Strict air cooling

By strict air cooling, I mean no radiators. If all your components are cooled by air, you should normally have your top fan(s) as exhaust(s). But, this is not because of convection! This is simply because the component that is the nearest from the top fan(s) is the CPU, a major heat source. Therefore, you wan't the heat to be taken away from the heatsink as fast as possible. Also, this is a good way to keep directional airflow. The top fan will create a low pressure area near the CPU heatsink area which will cause the low pressure area to be filled by the closest source of air. That source should also normally be an intake fan providing cool air for the heatsink's fan(s). If you have a radiator on the back fan, they should always be set up as an intake for the best amount of air. Usually, they will be single radiators hooked up to a CPU cooler like the H Series from Corsair. Therefore, they are cooling the major heat producing component in that area. Because of that, the slightly heated air from the radiator will not cause the temperatures of any components to rise because there's nothing to cool. Of course, there's the VRM area that produces heat but it's a negligable source of heat and could probably be cooled passively in most cases (ie: anything but extreme overclocks on high TDP CPUs).

Now we have seen where your top fan(s) should be set up as exhausts. Sometimes though, you wan't them to be intakes and this is when most misleading advices come.
Rather than words, the best way to test this is with numbers. It's simple and rather concrete. I don't mean to sound bitter, but quite honestly, this test can be done qualitatively, and recommendations should be based on numbers.

I tested my case just now, once with a top intake fan (140 mm) mounted in the forward position (toward the front) then once with the fan in a top exhaust position, mounted in the rear. In both tests, the other side was blocked with a sheet of polycarbonate.
I let my computer idle (well, I'm watching a SC2 stream) for a few minutes, then ran Prime95 for 6 mins (1344 FFT, 5000 MB RAM, 1 min interval) while recording temperature data with realtemp 3.69. I averaged the temperature across all 4 cores, then took the average of these 'average core' temps, starting 10 seconds after beginning Prime95 each time. The results are as follows:
Top intake 54.9 ± 1.1 ºC
Top Exhaust 56.2 ± 1.1 ºC
Ambient temperature was the same for both (I have a thermometer right here), 20.7 ºC
As can be seen, it's rather minimal. But, the effect is there, at least for me in my case with my setup. An intake fan is more effective for cooling. And now that we know the answer, we can begin trying to explain it. I think that the intake fan brings fresh air directly into the CPU cooler. With an exhaust fan, I think that there's slightly less air available for the cooler, if that makes sense. Either way, these are my numbers, if people want to do their own tests, even better, because more data is ideal.

In the spoiler below is a picture of my case, for completeness.
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I've been saying for years that hot air won't rise if you have a fan(s) pushing it. I've had my case fans in every combination and top intake is the best option for my system.
 

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Ever since I had my phantom, I've always ran both top 200mm fan as intake and it does wonders for the vrm and ram temps along with every other air cooled component on the motherboard. I can't comment on anything else since the rest is watercooled with an external radiator. A fan pointed at naked ram dimms is vastly better than any fancy heatspreader can accomplish if the top fans were exhaust.
 

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the importance and relevance of convection cooling is often OUTSIDE the box.... to discount convection entirely is uneducated at best... yes inside a box, any kind of fan would overpower the action of convection, however, this does not negate for physics behind heat rising to the top. with a top side intake, you are often recirculating hot exhaust from PSU/GPU/CPU right back into the case and causing further inefficiency in your overall cooling solution. this problem is especially acute if the computer is in a semi enclosed area such as under a desk where hot exhaust is trapped by the desktop and then sucked back into the case from the top side intake.

recirculation of hot exhaust (thermal short circuit) creates inefficiency and as a result higher decibel levels then necessary to achieve the same goal. overall cooling performance may not be very noticeable however the real discussion in PC cooling really lies in long term decibel level. anyone can cool anything if they used enough 5000rpm delta fans, however, to keep it under acceptable decibel levels, one must search for efficiency and this is where consideration in convection comes into play

a bottom intake and top exhaust ensures maximum separation between cool intake air and hot exhaust air because OUTSIDE the box, hot air WILL rise and convection DOES work...
 

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Discussion Starter · #11 ·
Quote:
Originally Posted by psyclum View Post

the importance and relevance of convection cooling is often OUTSIDE the box.... to discount convection entirely is uneducated at best... yes inside a box, any kind of fan would overpower the action of convection, however, this does not negate for physics behind heat rising to the top. with a top side intake, you are often recirculating hot exhaust from PSU/GPU/CPU right back into the case and causing further inefficiency in your overall cooling solution. this problem is especially acute if the computer is in a semi enclosed area such as under a desk where hot exhaust is trapped by the desktop and then sucked back into the case from the top side intake.
recirculation of hot exhaust (thermal short circuit) creates inefficiency and as a result higher decibel levels then necessary to achieve the same goal. overall cooling performance may not be very noticeable however the real discussion in PC cooling really lies in long term decibel level. anyone can cool anything if they used enough 5000rpm delta fans, however, to keep it under acceptable decibel levels, one must search for efficiency and this is where consideration in convection comes into play
a bottom intake and top exhaust ensures maximum separation between cool intake air and hot exhaust air because OUTSIDE the box, hot air WILL rise and convection DOES work...
I know the thread is very old but I just wanted to answer your comment. The air outside the case doesn't matter either for these reasons:

1) If you have your case in an open space, the amount of air is so large that the probability that exactly the same air is pulled back into the case is minimal at best. Anyway, because of the laws of thermodynamics, the heat would have a tendency to dissipate itself into the rest of the air to maintain a certain balance.

2) If your have your case in a small space like the little compartments that they put in many desks, air will be recycled anyway and you should just get it out of there if you want better temps.
 

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Most cases will have one or more airflow obstructions near them... chair, ball, desk top, human leg, etc. The resulting contamination of cool intake air by hot exhaust air will be the result of how these obstructions interact in airflow.
 

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I dislike the idea of having the top of my PC for intake for one reason and one reason only...

Dust....
 
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Discussion Starter · #14 ·
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Originally Posted by Ganf View Post

I dislike the idea of having the top of my PC for intake for one reason and one reason only...
Dust....
Of course that's something you have to take into account but some cases have top dust filters like the HAF X if I'm not mistaken but cooling wise, a top intake is often better than a top exhaust especially for watercooling.
 

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Quote:
Originally Posted by Snowmen View Post

Of course that's something you have to take into account but some cases have top dust filters like the HAF X if I'm not mistaken but cooling wise, a top intake is often better than a top exhaust especially for watercooling.
No filter is going to catch it all though, unless you let it get so clogged up with pet hair that it can't be seen through.
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and it's the fine dust that kills components. The chunky stuff is ugly sure, but it's not going to get into your ram slots, the bearings of your fans, or the back side of the board. It's the talcum powder fine dust that will end up costing you money, and the only way to keep that out is by setting your intake low and your exhaust on top, with positive pressure. Or don't get a case with an open top.
 

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At the moment, i only have one active fan (NOCTUA 120mm @ 1500rpm) on my corsair 300r and that is on my antec 620 radiator as a push exhaust on the top of the case.. I did some tests and see no big difference on performance with more fans on the case.... But i guess people here thinks more fans are better... So be it.. XD

The rest of the cooling is been doing passively and using the case aerodynamics to my advantage..
The only fan i hear now is the gpu fan and i would kill that one soon too...

http://www.overclock.net/t/1267656/aircooling-test-for-gurus-xd/10

I dont like theorys i like actual facts, hence my tests and their results with graphs..

What ifs dont count..... Test your system.. More fans dont equal better performance...
 

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Discussion Starter · #17 ·
Quote:
Originally Posted by zGunBLADEz View Post

At the moment, i only have one active fan (NOCTUA 120mm @ 1500rpm) on my corsair 300r and that is on my antec 620 radiator as a push exhaust on the top of the case.. I did some tests and see no big difference on performance with more fans on the case.... But i guess people here thinks more fans are better... So.. XD
The rest of the cooling is been doing passively and using the case aerodynamics in my advantage
http://www.overclock.net/t/1267656/aircooling-test-for-gurus-xd/10
I dont like theorys i like actual facts, hence my tests and their results with graphs..
What ifs dont count..... Test your system.. More fans dont equal better performance...
That's probably because the Corsair 300R is pretty much made out of mesh. Your two main heat producers, the CPU and the GPU, are properly cooled because your 620 has a fan on it and the GPU is surrounded by mesh so it's almost as if it was running in the open (like a test bench).

The number of fan matters but only if you place them properly. Also, if you have that 120mm fan exhausting, the gpu also exhausting with a blower style cooler and your PSU facing up, these are all taking air out of the case. Now, the case can't simply be "filled" with void so cool air will eventually get in.
 

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Like i said everybody need to do their own tests to find out.. if you think more fans are better just to have a ease of mind fine with me..
I even block my side panel mesh with cardboard to do this tests.. My findings? Only 1 fan needed...
My case only open is the top which is good for me it helps me alot XD...

Less fans, Less dust on my components..
 

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Sorry this is wrong. You should not have you radiator doing air intake. I have done extensive testing with several cases and radiator combinations and always the exhaust radiator improves thermal performance. While intake radiator leads to un-wanted side effect as higher motherboard temps, hdd temps, and gpu temps.
 

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Quote:
Originally Posted by Acefire View Post

Sorry this is wrong. You should not have you radiator doing air intake. I have done extensive testing with several cases and radiator combinations and always the exhaust radiator improves thermal performance. While intake radiator leads to un-wanted side effect as higher motherboard temps, hdd temps, and gpu temps.
Dude you realize im maxing out on IntelBurnTest (IBT) @ 70/72c (14800mb ram loaded on a single test) for maximum constant heat to find out my cap, Thats almost 8minutes of constant abusing NON STOP with a non-realistic load... You cant find nothing out there that abuse your cpu like IBT/LINPACK 64, i guaranteed you that..

I dont really care in the couple of extra -c im going to gain putting my fan on the radiator as a intake... It dont benefit my overall system at all, i think it would get worst as it pushing that heat inside.....

Theres no way, no program out there load my cpu as much as IBT not even prime with @ 12hr test would do that...
Guess what, i only need 8minutes of IBT to do something prime95 cant do witth 12hrs....
 
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