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noobie question on which direction the cooling fan should blow

B

Bowmaster

Senior member
OK, I've assembled my first system, and it appears to be running fine. One question I had during assembly was this: should the cooling fan for the CPU suck hot air away from the heat sink/CPU or blow cool air towards the heat sink/CPU? The picture on the heat sink showed the fan sucking hot air away for the heat sink/CPU, but I have read in different forums that the fan should blow cool air across the heat sink. I can rationalize both types of operation, and just wonder what the Anandtech consensus is.

btw I'm using a Thermalright SK6 with a AMD1.4
 
Welcome to Anandtech.
Most people will say have the fan blow down on the HS. It my not even perform well at all if you have it 'sucking' up. Good heat-sink.
 
Okay, so although I may be wrong, this is my opinion-

Heatsinks cool things by radiating heat away from the base to the fins and then to the atmosphere.

Apparently with some input from my sister who has done extensive thermal studies @ the University of Geneva in Switzerland, the best way to mount and cool a CPU is to have it mounted flat perpendicular to the force of gravity - just as you were laying it flat on a table. Mount a standard heatsink with fins facing upward, and your heat will be radiated along the heatsinks upward towards the sky (according to the laws of physics, heat rises).

If you would mount a fan, again this is my opinion, the best way to do it is to mount the fan to blow upward away from the heatsink in order to assist the heatsink in radiating the heat to the atmosphere via natural convection. Adding a shroud or a 'skirt' around a portion of the heatsink will theoretically draw air in the channel closer to the base of the HS, thereby covering more surface area.

If you totally disagree with my input, PLEASE correct me, I'm interested to know....

 
DongTran:
Your point certainly is valid and I am far from being an expert to tell you otherwise. However, you mentioned adding a channeling skirt which could be considered a heat-sink mod albeit an easy one. This particular HS does not seem to do well with the fan sucking. In fact, I have only seen one report to the contrary and my own trials with the SK6 have yeilded better performance with the fan blowing down. This may totally be opposite with other HSs. An added benefit of blowing onto the board is the wash air is moving down and over nearby components giving them some circulation as well.
Bowmaster:
Give 'em both a try and report back your findings😛
 
Thanks for your replies so far. I'll try it both ways and report what the temperature is upon bootup.

My only worry with having the fan blow cool air over the heat sink is that the hot air will be blowing directly onto the mobo, whereas with sucking the hot air away it will just go into the case and be carried away by case fans. And as I said, the picture that came with the heat sink showed the fan mounted on the heat sink so that the hot air was being sucked away.

I'm not overclocking, so it really isn't an important point for me - I just know that my temps are "too high" (higher than they should be, still in the upper 40s) and I suspect that it is because I'm sucking and not blowing (hehe).
 


<< Your point certainly is valid and I am far from being an expert to tell you otherwise. However, you mentioned adding a channeling skirt which could be considered a heat-sink mod albeit an easy one. This particular HS does not seem to do well with the fan sucking. In fact, I have only seen one report to the contrary and my own trials with the SK6 have yeilded better performance with the fan blowing down. This may totally be opposite with other HSs. >>



Hey thanks for the feedback. I really don't know what the SK6 looks like, but I know that the heat shroud is used on many applications stock, such as almost all of the Alpha heatsinks and the Thermaltake Volcano 7. I know that for the thermaltakes, all of them blow down towards the HS, I'm pretty sure they have a reason for doing that way, I just switch the way the fan is blowing. I guess I'll some testing tonight and post my findings? I'm using a Volcano 7 on a XP1700 right now. I guess it'll be interesting to see what happens when I change the postion the fan is blowing. I also noticed that when the fan is blowing away from the HS, it is a lot quieter than vice versa.....just a thought....
 
OK, I tested my system last night (after running the system for about an hour under moderatly heavy load):

fan set to suck hot air away from heat sink: 54C
fan set to blow cool air onto heat sink: 47C

Another added benefit is that the fan noise appears slightly lower. Lower fan noise and lower temps = good (duh).

Thanks for your guys help.

 
just a note, this isn't a universal "solutioN" to this frequently asked question. For most heatsinks, yes, blowing onto performs superior to "sucking" air off of them.

I can see why an Sk6 wouldn't be nearly as effect sucking since its got minimal space for airflow....

However, there is hte notable exception of heatsinks that often perform better "sucking", save in cases where there are airflow conflicts: The alpha's.



Mike
 
ok, this may be a dumb question, but what about blowing sideways? What I mean is, don't mount the fan on top of the heatsink, but rather, mount it on the side of the heatsink and have it blow across the heatsink. Has anyone done anything like this? I was just wondering, because the idea occurred to me that I have 2 spare Panaflo L1As, and maybe I could mount both of them onto the heatsink on the sides, across from one another. I'd do it so that they would both be blowing in the same direction: the one fan would be blowing air across the heatsink, while the other fan would be sucking the air away from the heatsink.

whaddaya think? stupid idea? Or might it just work? Has anyone tried anything like this (or similar)?


-hudster
 
Dude, blowing is better than sucking...I thought you knew? 😀

Blowing will create a high pressure zone on the side where the air is blown. Here will be more dense air. "Thicker" air can ofcourse carry more heat away. This is why blowing is better than sucking. Flow being equal, you want the high pressure to be in the area where the heat is.

Now as for heating the rest of the MB, thats another issue.

Same for cases, If I had only 3 fans I would have 2 fans blowing in, 1 blowing out.

Mention it to your sister.
 
WA261, Dong has already covered your POV. 🙂

My understanding is that the pArTiClE Theory states that heat will always transfer from a warmer substance to a cooler substance, and stop when both are the same temperature. So internally, a heatsink's fins/pins will continue to conduct heat regardless of airflow. ?
 


<< Doesnt alpha recommend that the fan suck air off of their heatsinks such as 8045??? >>



You know it does, but it's wrong. As with many things on that page I suspect the translation got hosed. (8045 is about 3C cooler on blow, typically)

Blow is always better unless you use a cowl of some sort to create negative pressure behind the fan. Just think of it like this... when you're hot do you sit in front of a fan or behind it?
 


<< You know it does, but it's wrong >>


That's why the 8045 gets better temps when the fan is sucking, right? It has a shroud, you know.


<< 8045 is about 3C cooler on blow, typically >>


I might be a little ignorant here, but I haven't heard of an owner whose temps were cooler on blowing.
 
Haven't tested it in windows yet, but when I turned the fan to blow instead of suck on my SK-6 on my other system, the temp on the cpu went from 46 degrees to 33.
 
Hum I have my sunon blowing into my alpha 8045...maybe I'll change it around tonight and see what happens.
 
Blowing down on the cpu is probably better. The biggest concern is that you are moving enough air through the case. The newer cases that have 2 fans on the back pulling air out of the case near the cpu seem to work really well. Try to have all the air come in the front of the case, and all of the air go out the back of the case.
 
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