How to position these case fans?

[DJ Bobo]

Hi guys,
I'm replacing my old noisy fans with 2 new silent case fans: one with a ventilation volume of 26 cubic meters per hour, and one with 45 cmph.
One fan will replace the old fan of the power supply unit (get the hot air out of the case) and one will work as a front fan (get fresh air into the case). Since this is an elderly case, I don't have any other possibilities to place fans.
Where should I place the stronger fan in order to maintain temperatures as low as possible? in the front area or in the PSU?
The case is a midi-tower.

[CWR03]

It's always my preference to have more air blowing in than being pulled out, and I filter the incoming fans to keep the case clean inside. I just built up a Coolermaster Cosmos case and flipped the two top fans and added filters - its design was to have them pulling air out the top, but a friend who has the same case has tons of dust getting trapped in the aluminum mesh in the front.

When moving air, it's a good idea to make sure there's flow around the hard drives.

[Dr.Khron]

I'll disagree - I generally prefer to have more pulling out then in.

BUT, this depends on the flow in your case, and your hard drive cooling.

EDIT:
Insense sticks are occasionally useful for troubleshooting airflow problems. Just make sure you buy the thick stinky ones that make a big smoke stream.

[CWR03]

There were several other reasons why I flipped the top fans on mine. Another is that there's a vent and a plastic shroud to pull outside air over the video cards, but the vent is right next to the exhaust on the cards. The vent was just pulling the hot air right back into the case, so I tossed the shroud and flipped the fans, which dropped the temp on the cards by 5° C. The vent is now exhausting hot air from around the cards.

[DJ Bobo]

CWR03, thanks for the advice about the filter in the front area, I think I'm gonna get one, one's never too careful with dust.

Dr Khron, my case is pretty much simple. Old concept, so no cooling for the HDD, it can hold only one fan in the front area, which is below the HDD cage, so the front fan is blowing air directly to the lower mainboard components (Southbridge and PCI slots)
The PSU is on top, and has air vents in the rear part (not below).

Now, the question remains: where to put the stronger fan? I'm a little confused now since you two have completely different opinions on the matter.

[GrofLuigi]

Now, the question remains: where to put the stronger fan? I'm a little confused now since you two have completely different opinions on the matter.

It's mostly about dust getting in or not - positive or negative pressure.

One link (http://www.tomshardware.co.uk/forum/238184-13-case-airflow-positive-negative-pressure), another (http://episteme.arstechnica.com/eve/forums/a/tpc/f/77909585/m/916000015831?r=916000015831)...

GL

[LoRd_MuldeR]

I'd put the stronger one in the PSU, just because an overheating PSU is very dangerous.
In case the PSU blows up, it may kill all the other hardware along with itself :scared:

[DJ Bobo]

Thanks GrofLuigi very much for the links. They really helped! I think, I'm going to put the stronger fan in the PSU, as suggested by Dr Khron and Lord Mulder. Someone pointed out in that Tom's Hardware forum that most case designers would put more exhaust fans than intake ones, so it seems like negative pressure is the better solution. Now that I think of it, before CWR03 flipped anything, he had 4 exhausts (including the PSU one) and only 1 intake! And we're talking here about a high-end chassis, so it pretty much speaks for itself.
The point made by Lord Mulder about the PSU getting hotter with a weaker fan seems also valid to me, I really don't wanna loose another computer (I've blown up a P4 already!)

[CWR03]

The Cosmos is an odd design - one fan just circulates air around the hard drives and doesn't really pull any in, even though it looks like it does; the PSU is in the bottom of the case and pulls air in from outside (and through a filter) and exhausts it out the back; only one now exhausts, and the two on top pull air in. My old case has two filtered fans in front and only the PSU pulling air out.

I don't know how important it is to keep the inside of the case clean, but I've always done it this way and have multiple computers that are over five years old and have been running 24/7 with no hardware failures.

[DJ Bobo]

@ CWR03
Oh yes, sorry, I thought the PSU had an exhaust fan, my bad. Still, the original design would be 2 intake fans at the bottom and 3 exhaust ones at the top, so this is definitely a negative pressure design (also makes sense when considering that cool air stay below and hot air naturally goes up)
Meanwhile, I went to the Cooler Master site, I checked the specs of more affordable cases, the Centurion 5 for instance, its original design has an 80mm intake in the front and a 120mm exhaust in the back. So this is a negative pressure design. The PSU exhaust only emphasizes the negative pressure design.
Actually I wonder if it really was a good idea for you to flip the top fans in your Cosmos case, I mean you have that rear exhaust of yours that is really close to these top fans. So may be you just cannibalized the exhaust fan and the top fan close to the edge, creating an exclusive wind channel between these two, I don't know. Sounds fishy anyway.

[CWR03]

I thought that too at first, but all that air flows right across the processor. The air exhausting through the one rear fan is quite warm, so I know it's not just pulling it in and blowing it right back out.