[SOLVED] Which Way Round for Fans?

[Oasis Curator]

I've just installed a Noctua NH-L9i CPU cooler.
I've noticed the fan spins so air is pushed towards the CPU.
I always thought fans drew air away from the CPU. However looking at my main PC, it appears the stock AMD fan also draws air towards the CPU.

If that's the case, should my case fans pull air in or push air out?
The main PC has a through flow but this new one won't, just one fan at the rear.

 

[USAFRet]

Pushing through the radiator fins.

In this case, down towards the CPU.

 

[Eximo]

Your rear fan should be an exhaust fan or going the same direction as your CPU cooler.

Typical case design is intake at the front/bottom exhaust at the back/top.

If you have no front intake, that means every unsealed location becomes an intake. This can lead to dust buildup in nooks and crannies.

 

[USAFRet]

For the case fans, you ideally want a front intake as well.
To facilitate air moving through the case.

 

[Oasis Curator]

There is a fan slot on the side next to the CPU cooler, so I could pop one fan there to pull air in and grab another fan to push air out the back.
But the case fan heights are very nearly at the same height so not sure if the fresh air coming in will get pulled down by the CPU fan or just straight out the back. There are two slots at the front for fans but the shape of the case / location of components, not sure they'd work that well. The previous iteration just had two fans out the back and that was that. It wasn't to bad for dust over the 10 years of life it's had.

Perhaps I'll just do some testing - mount one in each place (1 x side, 1 x rear ) and watch a film.
Wait a day, then mount both at the back and see what temperatures are like.

Don't know why, I was always under the impression fans pulled air away from the CPU to cool the fins.
Wow, that's like 20 years of wrong!

 

[SkyNetRising]

There is a fan slot on the side next to the CPU cooler, so I could pop one fan there to pull air in and use the back to push it out.

Please show a photo of your system with side panel removed/all fans visible.
(upload to imgur.com and post link)

 

[Oasis Curator]

Please show a photo of your system with side panel removed/all fans visible.
(upload to imgur.com and post link)

At work at the mo but here's a photo from the net:

At the front left, that large metal holder is removed as I don't have a optical drive.
The middle cage for HDD is also removed as I only need mounting space for one.
There are two fan slots behind the two cages at the front (for front intake, although the last iteration of this PC didn't utilise them).

I've removed the two fans at the back in preparation for adding two new ones that are likely to be quieter (if they're not, it's a simple change back).
The only problem is that if I don't have the two at the back, there would be a large gap.

The CPU sits next to that side fan slot. I have a low Noctua fan providing cooling there.

There is no graphics card to worry about on this build either.

 

[SkyNetRising]

That's a terrible pc case. Not designed to house gaming grade components inside.
Doesn't realy matter, what fans you put inside. There is no air-flow. No dust filters. Only small and loud fans fit.

I'd get rid of it and get a modern pc case instead.

 

[Paperdoc]

I agree that case layount is poor. All air flow from side intake to rear exhaust fan pair is short, with little flow elsewhere. Might be better with the front intake fans in place. We have a similar case we use for a PVR system - NOT a high-power gaming use! But the layout is different, so the rear exhaust port is away from the CPU and we have an intake fan on the side further from the CPU. The case TOP has a grille of holes placed just above the CPU location so the CPU cooler fan actually draws air in from outside the case and functions as part of the overall air intake. Unfortunately there is no dust filter on that grille, but dust accumulation has not been a big issue.

 

[Oasis Curator]

Sorry, perhaps I should have mentioned, this will be a HTPC.

I've used a gaming motherboard as I needed the umph for 4k.

To be fair though, I ran it for a bit yesterday and the processor didn't move much past the 50°c mark so maybe only one fan at the rear will be required.

 

[tennis2]

Don't know why, I was always under the impression fans pulled air away from the CPU to cool the fins.
Wow, that's like 20 years of wrong!

All Intel and AMD stock coolers and GPU coolers have their fans arranged to blow air down onto the CPU. You can generally tell the airflow direction of a fan by the open side, which is the side air is entering. The exhaust side has splines that hold the motor and such.

CPU air coolers generally have 2 open sides. More if it's a radial fin arrangement like the Intel/AMD stock coolers.
Fans inherently have the greatest airflow at the tips (outer circumference) of the fan blades.

Because of the open sides of an air cooler, if you're trying to pull air through them, the path of least resistance is to suck air in from the open sides of the cooler rather than the opposite end. Because of this, you're getting less airflow depth/uniformity through the cooler compared to pushing air into the fins. Pushing air into the fins ostensibly results in air leaking out the open sides as well (depending on the flow pattern of the fan, see below) but you generally lose less in a push scenario.

Downdraft (<= ) air coolers like the Noctua NH-L9i mentioned here also have the added benefit of cooling the mobo VRMs with the air that splashes out against the CPU and radiates outward.

Ways to combat the efficiency losses of pull config: Enclose the open sides of the fins. Much like a water cooling radiator which has all 4 edges enclosed, if the air cannot escape out the sides of a cooler, it has to pass all the way through the fin stack regardless of whether you're pushing or pulling.

###To be clear, we're not talking MAJOR differences in cooling performance between push and pull configs on air coolers. Most of them are relatively thin (compared to something like the Sycthe Mugen 5 or similar) so the efficiency losses aren't dramatic. BUT, unless a push config creates clearance issues, it's the best for air coolers.