What way should my fans blow Fractal Design Define S

joshuapaul433 · Dec 2, 2015

Title says it all really. I had the back fan blowing air out and front blowing air in. But I swapped them around. My system is:

Corsair RM750(sits at the bottom)
GTX 970 SLI
I7 4770K+Stock Cooler

2 Fans at the front blowing out. 2 fans blowing air in at the back.
I swapped the fans round because my idle temps were around 45/50 but I realised that I didnt put enough thermal paste on. So wondering if it is better if I change the fans back?

Thanks!

EDIT: I swapped my fans back around and removed the 3 extra fans I had(I had one smaller fan unplugged) and my temps are the same 28 degrees idle. So I am happy!

Outlander_04 · Dec 2, 2015

in at the front and bottom , out at the top and back

timeconsumer · Dec 2, 2015

I like to stick with the ATX specs and have the front be the intake and the rear exhaust. But you can always test it and see which is giving you better temps, every case is different.

Dugimodo · Dec 2, 2015

Heat naturally rises so it makes more sense to extract it from the upper parts of the case. Rear and top fans are generally higher than front fans so work better blowing the hot air out. Sometimes turning the airflow around can work better, but usually it doesn't. Also extracting the hot air is generally much more effective than trying to blow cool air in.

In some cases I've owned the front intake fans actually have 0 impact on overall system temps and only serve to cool the front mounted hard drives a little. This is presumably because even without intake fans the same amount of air is leaving the case and has to find it's way in anyway, all the intakes really do is choose where the air comes from.

A bit of a sidetrack but for positive pressure to work well (where more air is sucked in than blown out) the case needs to be designed for it but negative pressure (more air blown out than sucked in) always helps temps provided the hot air is extracted high up. That's why cases usually have a rear extractor but don't always bother with an intake. The only real reason to use a positive system is when you filter all the intakes as it prevents dust accumulating. Negative pressure means you need to clean the dust out periodically.

timeconsumer · Dec 2, 2015

^ Good analysis. Although one other benefit to positive pressure is for those using reference cooler GPUs. The extra pressure helps the GPU to blow more air out the back of the case.

joshuapaul433 · Dec 2, 2015

Thanks for the answer's guys I think I shall swap them back to what they were originally and test my temps

Outlander_04 · Dec 2, 2015

Dugimodo :

Heat naturally rises so it makes more sense to extract it from the upper parts of the case. Rear and top fans are generally higher than front fans so work better blowing the hot air out. Sometimes turning the airflow around can work better, but usually it doesn't. Also extracting the hot air is generally much more effective than trying to blow cool air in.

In some cases I've owned the front intake fans actually have 0 impact on overall system temps and only serve to cool the front mounted hard drives a little. This is presumably because even without intake fans the same amount of air is leaving the case and has to find it's way in anyway, all the intakes really do is choose where the air comes from.

A bit of a sidetrack but for positive pressure to work well (where more air is sucked in than blown out) the case needs to be designed for it but negative pressure (more air blown out than sucked in) always helps temps provided the hot air is extracted high up. That's why cases usually have a rear extractor but don't always bother with an intake. The only real reason to use a positive system is when you filter all the intakes as it prevents dust accumulating. Negative pressure means you need to clean the dust out periodically.

Doesnt it logically follow that if you are blowing air in from the front and bottom then the same amount of hotter air must be displaced from the case?
Thus there should be no difference in cooling between intake and outlet fans?

Dugimodo · Dec 2, 2015

Well you'd think so, but it's not as efficient. Unless you completely seal the lower portion of the case some air escapes wherever there is a gap because you are slightly pressurising the case. Pushing cool air in can and does work, but extracting hot air from were it naturally accumulates works better. It's just fan assisting what will happen anyway. intake fans do not guarantee the hot air you want to move actually will, air currents can do weird things.

Positive pressure systems are better overall when designed right, that's what you will find on a larger scale in data centres for example, but they are normally less efficient and noisier. If you lined the front and bottom of your case with well filtered high airflow intake fans you can get very good performance and a dust free interior. If you don't filter them you blow dust etc through the whole case even worse than normal.

A good balance of both is what you get with a front intake and a rear outlet, it works well because you are just adding to the natural airflow. If you reverse it you are fighting the airs natural tendency to rise as it's heating and expecting the hot air to be sucked down and out the front. Some of it will instead rise and maybe escape or maybe find a quiet spot with no airflow to accumulate in.

timeconsumer · Dec 2, 2015

I also never add fans to the side of a case. I find it actually increases temps. Most likely caused by it creating turbulence and not doing much to actually help get air to the right places and out.

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What way should my fans blow Fractal Design Define S

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