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HSF (heatsink fan) question

M

MaxPower

Limp Gawd
Joined
Nov 8, 2001
Messages
191
To me this has been a question that has plagued me ever since i started building systems. What is the more preferred way to place the fan on the CPU heatsink?

I've always placed them so the fan would exhaust the air off the heatsink. But whenever i tell some people this they look at me like i have two heads. I guess they've always been told to do it the opposite way.

My thinking is if you're trying to get hot air out of the case, then why blow the same recycled hot air down on the already hot heatsink? (i know many people place intake fans on the side panel paralell to the heatsink). But i say if you get the hot air out of the case first. Then the heatsink fan can aid the heatsink more by pulling the hot air away from it.
 
that depends on the heatsink, now doesn't it?

stock coolers, and low-end aftermarket coolers have the fan installed the way that you are doing it, and that is the way that they are supposed to be.

those low-end 'sinks tend to have the bases of the fins very hot, and the tips fairly cool. by drawing air in, through the fins, and out the fan, it gets cool air to the only part of the heatsink that is actually getting hot, and cools that area down.

on GOOD heatsinks, whatever the material, the whole fin gets hot. that means that having the fan blowing down onto the fins will usually give best performance. the air gradually gets hotter and hotter as is goes, and picks up heat along the whole trip, then leaves.
 
i prefer to place the fan so that air is blowing down on the heatsink. you would think placing the fan to draw air out of the heatsink would cool the heatsink better but that isnt the case. silentpcreview tests their heatsinks with the fans turned both ways and here their review of the thermalright xp120. in the table they show cpu temperatures with the fan turned both ways.

http://www.silentpcreview.com/article186-page4.html
 
MaxPower said:
My thinking is if you're trying to get hot air out of the case, then why blow the same recycled hot air down on the already hot heatsink? (i know many people place intake fans on the side panel paralell to the heatsink). But i say if you get the hot air out of the case first. Then the heatsink fan can aid the heatsink more by pulling the hot air away from it.
Click to expand...

Basically the physics of it doesn't really work out that cleanly in either scenario, which is why your logic falls apart against the more popular convention, and also why it can vary depending on the heatsink/specific setup.

Either way you are just moving air past the heatsink and heating it (the air) up. After that the air just returns to your case. Its not really practical to suggest that a fan blowing away from a heatsink is directly pulling hot air from your heatsink, nor the opposite. Realistically its just making more air move through the fins. In both cases the air just returns to your case, which is where case airflow comes into play. If blowing air at the heatsink causes warm air to recycle, then so does pulling air from the heatsink. The method of pulling from the heatsink does make a little bit more sense when you observe normal case airflow (front to back) but the benefits are only slight. Blowing air AT the heatsink seems to be more efficient in forcing air to and through all parts of the heatsink, which, I think, is where the "from" method falls short: air can slip from the surroundings to and through the fan after passing just a few millimeters of heatsink (this would be at the very top edge of the heatsink). Air in the middle base of the heatsink isn't as forced as if it were being battered directly.

Anyway, that's just my take on it.
 
RawsonDR: have you done your own tests? without trying to get combative, i did some testing with a nice thermalright SLK 900 and an AMD stock cooler from a XP 2600+ desktop, and the results were as a i layed out. the explanation that i gave above was how i reconciled the results in my own mind, i just wanted to know your own perspective.

yes, i know damned well that i have far too small a sample to call this a real test, and my controls were not anywhere near good enough for doing any kind of formal writeup and conclusions.
 
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