Which way do (should) GPU / CPU fans blow?

[pointone]

I've searched around on the internet a bit, but couldn't really find a straight answer. Which way should the CPU / GPU fan be blowing: air away from the heatsink or air onto the heatsink? Is there a set standard? Which way is generally better, in your opinion?

Thanks!

[Deagle]

The fan should be blowing the air onto the heatsink. It's because as the sink draws the heat up, the cold air will come down and cool it again. All the heatsinks I've used the manuals all said the same way.

[Sylvander]

In Engineering [heat exchangers] it's called "Contra-Flow".
Heat transfer is most efficient when the cooling fluid flows in a direction which is the opposite of the direction of the flow of heat [thermal energy in transit].
This is because of the relative temperatures of the two, and their difference.
[Graphical representation is needed to best illustrate it.]
The heat begins at its highest temp and as it flows along the fins of the sink the temp drops [because it's tranferring thermal energy to the air].
The cooling fluid [air] begins at its lowest temp and as it flows in the opposing direction its temp increases [it's receiving thermal energy from the fins].
This produces a temp difference between the two that is approximately constant at all points along the fins which makes for maximum efficiency of heat transfer.

If the heat and air flow in the SAME direction:
The temp difference is very great at the beginning and almost nothing at the end, which is not so efficient. There isn't much difference in the two arrangements; the efficiency is only slightly better in contra-flow.

"Entropy" is the key to understanding why this should be, but I always had difficulty in fully comprehending what entropy is.

[slipbeezer]

Now I'm wondering too.... :)
Wouldn't the heatsinkfan already be attached in the correct direction when you buy it?

I recently bought a new, much larger one for my processor and there was no option for which direction to install it. And the other fans in the case can only go one direction too, I think. :confused: I would hope the manufacturer of the product knows the right way to install their product. Now I'm worried. The new case has 5 fans, pre-installed, perhaps not the best way.

From what Sylvander and Deagle said, a case fan blowing towards the heatsink is not good??? Doesn't seem like you can turn them around and be able to screw them on. (my first build and learning!)

[Whyzman]

Nearly all fans are setup by default in the "Contra Flow" position...something to do with entropy I think... :D

Preinstalled case fans usually pull air in from the front and push (exhaust) air out the rear of the case...You will normally see the setups moving air into the case from the bottom front and exhausting air in the upper rear using the natural phenomenon of warm air rising...

The more important question with placement of fans is air flow. Yes, volume of air does factor in, however, the cooling fluid (air) needs to be removing heat from the proximity of the heat producing devices. Or, they will use the heated air around them to expedite their own demise...

I would prefer to have good air flow in and out of the case with a slight positive case pressure (a little more coming in than going out). The reason for this is that if you create air flow with a negative pressure, you will draw in contaminants from every nook and cranny your case has to offer...

With a slightly positive air flow, I can use a filter placed behind my intake fans to help keep my computer's innards dust bunny free...

The only setup where I would strongly consider a same flow (pulling air through the fins) would be if I incorporated some sort of "blow hole" to immediately exhaust the CPU heat outside the case. One of our folks here, cannot remember who right now, customizes his cases by venting his CPU fan in such a manner.

[pointone]

Thanks for the information on the CPU / GPU fans... but now I'm worried about my case fans! lol

Right now, I have one exhaust fan at the rear and my PSU (also at the rear) blowing air out of my case. There are no intake fans. I had one on the side of my case, but it was far too noisy so I unplugged it. I'm not worried about the temperatures with this arrangement (although it always could be a bit cooler...) but now I'm worried about dust buildup. From what you say, it would be best to put a fan at the front of the case for intake, but there are no holes at the front of my case for one (well, there is a place to mount a fan at the front on the inside, but no way for air to get in.) I guess my best bet it to buy a new (quieter) side fan for intake, but I'm worried it makes the airflow in my case *worse.* When I unplugged the side fan I had before, my CPU / GPU temperatures *dropped* about 2 or 3 degrees C compared to what they were before (although I didn't run any "scientific" experiments to compare the temperatures.)

I don't know why else the temperatures would drop; the fan must be hindering airflow. What's strange about the fan is that it is in the exact centre of the side panel, while other (better) cases I've seen have a fan lined up with where the CPU / video card is. I really cheaped out on the case when I bought the computer, (I'm poor) and now I'm wondering if the dust is really a serious problem. Is there any way I could reduce the amount of dust in the case without buying a whole new one, or would that actually be the best option?

[Whyzman]

If your temps are stable but your case is pulling unfiltered air in, then it is important that you routinely inspect and clean the innards. It sounds like this would be a cheaper solution than purchasing a new case...