# CPU Fan: Pulling hot air off, or blowing cool air on?



## TheUnlimited (Dec 12, 2005)

Just wondering what's preferable for mounting a CPU fan. Should it pull the hot air off (pulling cooler air in through the fins), or should it blast cooler air down through the fins, pushing warmer air out the sides?

I'm asking only because my CPU fan just recently bit the dust, and I had to replace it. I noticed that it was set to pull air off of the heatsink, rather than blow down on it. Chances are that during a cleaning session, it was put on backwards. I've always thought that they were to blow air down onto the heatsink.

It's a nice, old copper Thermaltake Spark7, and kept my processor cool enough to run even without the fan (though I could do nothing beyond looking at websites).

Just curious. The new one's setup to blow on the heatsink. 8)


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## jnob (Nov 22, 2005)

It should blow onto the heatsink, it will move more air that way.


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## UncleMacro (Jan 26, 2005)

Nothin' personal, but I run fans sucking air through the heatsink all the time and I've never seen a temperature difference between sucking and blowing. In short, sucking doesn't suck.



I do it so I can rig a duct so the fan sucks cool air through the heatsink and blows warm air out the back of the case rather than exhausting it into the case. The big yellowish thing is the fiberglass air duct on the top of the heatsink.


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## TheUnlimited (Dec 12, 2005)

Thanks for the quick responses, guys. I'll leave it as it is right now, I suppose. ^_^

It's a bit noisy, since it doesn't auto-regulate the fan speed like the stock Spark7 fan. But at least I can DO things with the computer now. 8)

That's quite a setup you've got there, UncleMacro. Did you custom make the duct? I also see the Arctic Silencer Rev.3. Got one on my Radeon too. 

I would consider doing a little hot-rod duct work on the case, but it's not worth it at this point, considering I may build a new unit this summer.


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## UncleMacro (Jan 26, 2005)

TheUnlimited said:


> That's quite a setup you've got there, UncleMacro. Did you custom make the duct? I also see the Arctic Silencer Rev.3. Got one on my Radeon too.


Yea. I used to work with fiberglass a lot so I made it from that rather than (ugh!) cardboard. I've built CPU ducts for the last two of my computers. I like the idea of having the main heat sources (CPU and video card) exhausting their air out the back of the computer rather than leaving it inside the case. It's just dumb to vent your heat inside the case and then have extra fans to get it out. As a result, both machines stay at room temp inside the case and are fairly quiet considering their rather powerful cooling systems.


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## jnob (Nov 22, 2005)

Nice, I guess it just depends on the setup I've seen rigs that average 10C hotter when the fan is blowing away.


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## UncleMacro (Jan 26, 2005)

Well, I've done it on a grand total of two computers. I guess it depends on the heatsink. I've never seen anyone actually put it to the test on lots of different heatsinks.

Thermally speaking, I object to exhausting the hot air inside the computer. It just seems so dumb.


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## jnob (Nov 22, 2005)

I agree if I had the skills to put an exhaust such as yours together I'd be all for it. I'm sure that saves loads on your hdd, PSU, and chipset.


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## MunkyPhil (Apr 7, 2006)

When the fan is blowing cold air down onto the processor the temperature on the end of the fins is lower than it would be if hot air was being pulled up. This larger temperature difference between the processor core(s) and the finn tips allows a greater heat flow through the heatsink thus dissipating it faster. The physics of heat transfer and fin design cannot be argued with and the performance simply IS better when cold air blows down. If you haven't got a heat problem there's nothing to worry about though.


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## Volt-Schwibe (Jan 12, 2003)

munkyphil, i argued forever about this back in the day.

pics, and math, and the whole works wouldn't convince people of what you just said.

it was strange, like suddenly people couldn't grasp the idea that larger differences in heat cause _faster_ heat flow.

anyhow, i just gave up.


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## Tumbleweed36 (May 14, 2005)

Hi,

Wow, don't think there ever will be agreement on this issue, so here is my take on it.

First of all, in order for for a heatsink to be cooled effectively, it needs to come in contact with as much air as possible. Using a fan in reverse results in air taking the shortest route to the fan, thus the air *avoids* contact with the heatsink, especially at the center where the temps are greatest. 

By blowing air onto the heatsink it is possible to maximise contact so that heat is transfered to the air and then drawn away by the natural airflow of the case cooling. This is why all heatsink manufacturers have their fans blowing air onto the heatsink. 

As an example, imagine trying to cool a hot cup of coffee down so that you can drink it without burning your lips. Do you think that sucking air away, rather than blowing on your coffee, would have much cooling effect? No I don't suppose it would. You blow on it so that the cooler air (cooler than the coffee) lowers the temperature of the coffee. If you think about it, you blow directly in the center and not on the side of the coffee area. The center directed cooling provides the maximum cooling for heat dissapation

If you take a look at products such as CoolerMaster's Aero 7, and GlobalWin's CAK4-88T CPU Cooler's you'll find they have tried to maximise the amount of air directed towards the center of the heasink (plus using micro-fins to maximise air contact), and both have proven to be highly effective, and relatively quiet, coolers.

However, that is not to say that it isn't possible to produce a device that would work effectively with a sucking fan. That is quite possible, but its design would have to be somewhat different than the heatsinks that are currently used to cool CPUs. 

It is true that most heat is removed from the CPU by conduction via the heatsink, but the greater the temperature gradient at the centre of the heatsink, closest to the CPU core, then the more effective the cooling will be. As mentioned previously, reversing the heatsink fan will only reduce the temperature gradient, as the tendency will be to cool the outer edges of the heasink, which are already the coldest parts. 

Also, changing the direction of a fan or reversing it does not have any effect on its ability to move air, so it will likely be just as effective in cooling ability regardless of which way it goes around . The important factor is how cool air makes contact with the heasink at the point where it is the hottest.

Lastly, I need to mention that some tests have shown to cool more with the fan pulling hot air from the Heatsink, but it was later discovered in numerous tests that the temperature change was more the result of the change in ambient temperatures in the area of the sensor that it was actually cooling the center of the CPU itself. The focus of heat in a CPU is the center and that is why we use artic silver and place it in the center or the contact area for heat transfer.

That is my opinion on this issue.

Interesting thoughts:

The opposing view of this theory is that if heat (through a specially designed tunnel) is never permitted to enter the case and therefore draws cool air in to the bottom through the front of the case, the idea might have some validity. 

I do know that tunnels are designed for some cases (Antec Sonata II as an example) that provide cool air from outside the case to blow down on the heatsink. It does work (even without the two optional fans that can be placed inside the tunnel), because if you place a paper tissue at the back of the case where the tunnel inlets cool air, that paper tissue will be glued to the case by the air flowing from the outside of the case to the inside of the case. The cool air draw is significant and therefore, if the process were to be reversed, then the hot air draw would be quite significant also. Since one of my cases is an Antec Sonata II, I just might try that just for fun one of the days.

I don't want to sound like a politician on this, because I am still very strongly on the side of the school of thought that blowing cool air down is where it is at. This method IMHO is the most effective and efficient manner to cool the CPU/Heatsink combination, so I stand by my initial thoughts. 

However, having an inquisitive mind am willing to look at all posibilities. UncleMacro's theory does deserve some serious consideration and investigation as we try to find new methods to do the job in a more efficient way.


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## Tumbleweed36 (May 14, 2005)

Volt-Schwibe said:


> munkyphil, i argued forever about this back in the day.
> 
> Back in the day???????.You just be a kid yet!:4-dontkno
> 
> ...


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## Indoril Nerevar (Jan 7, 2006)

When a computer is (possibly) having overheating problems, one suggestion which pops up again and again is to open the case and point a desk fan at the open computer. Is this not the same concept as having a fan blow cool air onto a heatsink? Like the coffee analogy, when it's a hot day, do you find it cooler when you have a desk fan pointed towards you, or away from you? The best way to cool something down (other than submerging it in cold water) is to blow cold air onto it.


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## TheUnlimited (Dec 12, 2005)

Well, I didn't quite anticipate sparking such an interesting conversation. ^_^

I thank you all greatly for your insights. I wasn't aware that it was quite so hotly debated, but it's all quite fascinating. Thankfully, my CPU is a P4 Northwood @2.4GHz, so it runs cool already, topped with Arctic Silver 5 and a nicely lapped copper heatsink.



> When a computer is (possibly) having overheating problems, one suggestion which pops up again and again is to open the case and point a desk fan at the open computer.


That's actually something I've heard countless times. When Guild Wars was new, I remember that lots of people had serious cooling issues from the game's load...but they've since been (mostly) resolved.

Again, thank you all. 8)


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## UncleMacro (Jan 26, 2005)

*My heatsink flunked thermodynamics*

Since this has become such a controversial issue (many PC geeks are obsessed with cooling - I guess I shouldn't talk: I build ducts) I went back and tested my heatsink with the fan both sucking and blowing. When in doubt, measure. When not in doubt, measure anyway because you may not be smart enough to know when you should doubt. I think I built this machine about three years ago so my recollection is a bit fuzzy. My heatsink is one of those Swiftechs which has a big flat slab of copper with a bunch of aluminum pins stuck into it. I can't recall the model number. I use a 120mm fan with a fan adapter because I like things fairly quiet. These temps came from the thermal diode on the Pentium 4 die. (EDIT: This test was done with no ducts. The CPU fan was set up like a "regular" computer with the warmed CPU air venting inside the computer.) And the results are: [sound of a drumroll]

Fan sucking: 27C at idle and 44.25C while running Prime95.
Fan blowing: 27C at idle and 43.75C while running Prime95.
Perhaps you have a different opinion but I think 0.5C is a trivial difference. Maybe I've just got an unusual heatsink but I doubt it.



> As an example, imagine trying to cool a hot cup of coffee down so that you can drink it without burning your lips. Do you think that sucking air away, rather than blowing on your coffee, would have much cooling effect? No I don't suppose it would. You blow on it so that the cooler air (cooler than the coffee) lowers the temperature of the coffee. If you think about it, you blow directly in the center and not on the side of the coffee area.


That's not at all how a heatsink works. A heatsink has a large thick piece of metal to spread the heat from the heat source. It may also have heatpipes to spread the heat farther. The purpose of these is essentially to "move the fins closer" to the heat source. The farther the fins are away from the heat source the less effective they are so CPU heatsinks are designed to make the fins thermally as close as possible. Then it makes sure that lots of air is forced past the fins. Your coffee analogy would be the world's worst heatsink design with no attempt to either spread the heat out to lots of fins or force any air over the fins. If you plan to cool your CPU by having a fan aiming at the heat spreader on top of the CPU then you have a good analogy. But it's nothing like a real cooling system. In a real cooling system you do what you can to make the fins "thermally close" to the heatsource and then you force lots of air past the fins. Hence my suspicion that it's not just my heatsink which shows little difference between sucking and blowing. I think it's better to answer that question with testing rather than theories.

Regarding the PC cooling system as a whole, you need to distinguish between the major differences and the minor differences. Heatsink grease, for example, seems to elicit a bizarre amount of interest from overclockers everywhere. But if you do careful testing there's not much difference between them. Check here for an example of competent testing (and enough humor to keep an article about heatsink grease from being boring). There are differences between various heatsink greases but they're very minor. The primary thing is to use grease which doesn't dry out and doesn't cause other problems. It's the "other problems" part which has always made me avoid conductive/capacitive grease like the ones which contain silver. I used to design hardware (embedded process control stuff - nothing interesting) and if I had suggested using a heatsink grease which was even a little conductive or capacitive, someone would have smacked me in the head. It just isn't worth the trivial difference in heat conductivity to risk getting some goop on something which is upset by increased capacitance or a little conductance. If you're careful with that silver crap then you'll do no harm. But if you get it on a high-speed line you will cause serious problems. I've lost track of the number of people I've seen screw up their video cards when adding after-market heatsinks by getting Arctic Silver on the RAM bus pins. The extra capacitance increases crosstalk and slows the lines down. That causes video RAM artifacts. And removing the grease ain't easy. And all for a minor temperature improvement.

When it comes to cooling you should concentrate on the major stuff and ignore the minor stuff. I build with ducts because it's a major improvement in cooling rather than a minor one like kind of heatsink grease or (I suspect but have only tested a couple of heatsinks) rigging your CPU fan to suck or blow. The three main heat producers in a computer running at full load are the PSU, CPU, and video card. Your PSU already vents its warm air out the back of the computer. Both my video card and CPU (with it's sucking fan!) blow their hot air out the back of the computer. Most of your computers vent both their CPU and video card warmed air inside the computer.

When both your video card and CPU are loaded down, your computer design has placed a nice little 50-200 watt (depending on your hardware) space heater inside your computer. Even if you've got good airflow through your case, you're blowing warmed air over your heatsinks. A modern CPU fan moves lots of air at fairly high local pressures. Yours suck the air in from the top of the heatsink and blow it out the bottom. That sets up vortices which circulate your warmed air around the heatsink. Even with lots of fans moving air through your case, a portion of the air being blown down onto the heatsink is air which was just warmed by it. That temperature is not something which is measured very well by a motherboard temperature sensor.

My computer has its little space heater outside the machine due to ducting blowing the warm air from all major heat sources out the back. My warm air has to move all the way from the back of the computer back to the front of the computer before going in again. As a result I'm pulling nice cool room temperature air from inside my case over my video card and CPU heatsinks. My internal air temperature barely budges.

My design sucks internal room temperature air over my heatsinks because my space heater is in the room. Your design pushes internal warmed air over your heatsinks because your space heater is inside the computer. If you stick lots of fans in your case you may be able to get your internal air temperature near the CPU and video card close to mine but your CPU and video card fans will still be blowing internally warmed air. And you'll have to listen to the noise of your extra case fans which I don't have to have. I wanted extremely cool _and quiet_ so I wanted my CPU fan to both cool the CPU and be the main fan which moves air through the case. My video card fan also does double duty in the same way.

My "suck" system is cooler at a given noise level than a "blow" system because I paid attention to the big picture. If you'd like a cool and quiet system then the most important thing is to locate your little space heater outside the computer rather than inside it and make sure the air flowing over the heatsink is at room temperature.

_*So there!*_


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## Volt-Schwibe (Jan 12, 2003)

honestly, the duct is a major improvement like you are saying, so in that instance, i think it would work just as good either way.

however, in a "sealed" environment, i measured non trivial differences.

perhaps we need to just ask tom's hardware to scientifically determine the answer for us.

(i myself don't need convincing, since most heatsinks blow down onto the fins, i trust there is a reason for the entire industry to do things one way, and not the other.)


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## Tumbleweed36 (May 14, 2005)

> Fan sucking: 27C at idle and 44.25C while running Prime95.
> Fan blowing: 27C at idle and 43.75C while running Prime95.
> 
> Perhaps you have a different opinion but I think 0.5C is a trivial difference. Maybe I've just got an unusual heatsink but I doubt it.


To use your words, it is quite trivial. Otherwise, what you are insinuating is that neither of our theories is better than the other, because it makes no difference. That just might be true, if most computers had the duct system.



> Check here for an example of competent testing (and enough humor to keep an article about heatsink grease from being boring). There are differences between various heatsink greases but they're very minor. The primary thing is to use grease which doesn't dry out and doesn't cause other problems. It's the "other problems" part which has always made me avoid conductive/capacitive grease like the ones which contain silver.


Very Interesting information and I am going to explore that further. Seems also like your experience(s) have given you some insight into that area that most of us do not have. That is an excellent point that we all need to consider.



> I wanted extremely cool _and quiet_ so I wanted my CPU fan to both cool the CPU and be the main fan which moves air through the case. My video card fan also does double duty in the same way.


This is the part of your theory that I like most. I love cool and quiet, so have to do some more research on that one. 

I do have a 120mm blowing out the back and one 120mm blowing cool air in the front, both on the slowest speed that they can operate at. I am curious what fans and how many you are using in your case to achieve your quiet and cool atmosphere? My 3700 San Diego (my P4 2.8 sits about 2 degrees higher than my Athlon) is sitting here with those two fans on the slowest speed and running at 29c for the CPU and 31c for the system, so i am pretty cool on this end with my setup. However, I am using the duct system provided for my San Diego by Antec. When i get some time, might just reverse that heatsink fan to see what it says.

Hey, put that end quote back in there, I loved it, but honestly Uncle Macro, don't quit your day job because Jay Leno won't hire you after the quote about all of us being qualified to design heaters and not refrigerators, etc.. Maybe that is why you edited it, you realized that Leno would not hire you.:4-dontkno 

Thanks for the theory, I enjoy reading it and will do some personal research to test some of the theories you have put forth. Nice interesting discussion.


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## UncleMacro (Jan 26, 2005)

> To use your words, it is quite trivial. Otherwise, what you are insinuating is that neither of our theories is better than the other, because it makes no difference. That just might be true, if most computers had the duct system.


My test did not involve any ducts. I just ran it like a plain old computer like everyone else and got a trivial difference between fan directions. Maybe I've just aquired a couple of unusual heatsinks. It's hard to know without more testing.


> Hey, put that end quote back in there, I loved it, but honestly Uncle Macro, don't quit your day job because Jay Leno won't hire you after the quote about all of us being qualified to design heaters and not refrigerators, etc.. Maybe that is why you edited it, you realized that Leno would not hire you.:4-dontkno


Actually, the reason I removed it was because when I reread the entire thread, the comment came off as a lot nastier than I had intended.


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## Tumbleweed36 (May 14, 2005)

UncleMacro said:


> My test did not involve any ducts.
> 
> When I first read it, I thought it had said that you just tested it. I now see an edit in there about not using ducts that I might have missed, or maybe the edit came after I first read it, but doesn't matter anyhow......so that is interesting.
> 
> ...


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## UncleMacro (Jan 26, 2005)

> I am curious what fans and how many you are using in your case to achieve your quiet and cool atmosphere?


The only fans other than the PSU, CPU, and video card are a couple of extremely slow, inaudible 80mm Panaflos. I have one in front of the hard disks because I have three drives crammed together real tight and they got kinda hot. They were perfectly reliable without the fan but I just prefer to keep them cool because I'm more than a bit paranoid about heat. The other Panaflo is in the back mainly because the case has a fan hole and it was easier to plug it with a silent fan than find some cardboard to block the hole. I think I'd probably get the same CPU and video card temps if the only fans in the computer were the PSU, CPU, and video card. With all of them blowing hot air out the back of the case I get enough flow through the case as it is.

When I originally built the machine I never really seriously tested the thing to see if I could dump the Panaflos and get the same temps. All I did was test my CPU heatsink both sucking and blowing to make sure it was okay to build the duct. I just reran the ductless test this morning because I couldn't remember (48 year old brain cells - they suck!) the exact temperature difference.



> We have to keep this stuff light


Then please don't bring up BTX! :grin: Let's see... They put the CPU fan in the front of the case where you're most likely to notice the noise. Well, at least they made it 120mm. But if future CPUs start getting hotter again (that's clearly the long-term trend) then they'll have to use louder 120mm fans to keep it cool. If it was me, the last place I would put the CPU fan is at the front of the case. It's okay now but may not be in the long haul. And despite having a CPU duct of sorts it still exhausts all of its CPU warmed air inside of the case as usual. And because the CPU is placed at the front of the motherboard it's difficult to rig a _real_ CPU duct which exhausts out the back. Grrrrrr. This thread is starting to wander but maybe some of you system builder guys can tell me if my BTX contempt is justified (assuming you've ever built BTX).


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## MunkyPhil (Apr 7, 2006)

Volt-Schwibe said:


> munkyphil, i argued forever about this back in the day.
> 
> pics, and math, and the whole works wouldn't convince people of what you just said.
> 
> ...


Too true. I'm a qualified engineer and have studied thermodynamics extensively, that includes how to design heaters AND refrigerators :winkgrin:. I have studied fin design and heat transfer and can safely say, and I have the piece of paper to prove it, that I am right. :sayyes: :grin:

UncleMacro brings up the best point of all though by saying testing is better than theorising. His comments throughout this thread have shown that not every thermal system reacts the same, and none of that really matters if you have good temps. You still can't argue with the fundamentals though, UncleMacro's 0.5 degrees under load says that, regardless as to how trivial you all think it is.


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## Volt-Schwibe (Jan 12, 2003)

MunkyPhil said:


> You still can't argue with the fundamentals though, UncleMacro's 0.5 degrees under load says that, regardless as to how trivial you all think it is.


and, even if it was a trivial variation in that test, it wasn't in the tests i have done, both recently and in the past.

in some instances, it could even be more than a few degrees.

i have to 100% agree that a duct makes a difference in a few degrees, where as the direction of the fan is a difference of a few tenths degrees, same as brand of grease like was said.

honestly, in a situation with the duct on the cpu, and blowing out, you are partially sucking the air in a direction from the coolest parts past the hottest parts on the way out of the case, and so the ambient temp inside the case is far cooler as well.

i have a coolermaster case, and it has a duct, but it blows _in_ onto the processor, and i found that to actually worsen things.

long ago i took it out.

perhaps i should reverse my fans, and put that thing back in, blowing up off the heatsink and out the metal duct...

you inspire me with your duct, and your fiberglass skills make me jealous.

i'm not kidding, i can bondo, but bondo isn't the same


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