For years, PC builders have followed the “classic” airflow setup: pulling cool air in from the front (and sometimes the bottom), while rear and top fans push hot air out. But what if we’ve been doing it wrong all along, and a single top fan could be the reason your CPU is overheating and underperforming?
The big problem with setting all your top case fans to exhaust
Physics tells us that heat rises, so it seems logical to set all your top PC fans to exhaust, right? On top of that, conventional wisdom says each side of your case should be either all intake or all exhaust—you’re not supposed to mix them.
Wrong.
If you’re running an air CPU cooler, the front-most top fan can actually draw away the cool air coming in from the front before it ever reaches the CPU heatsink. In one fell swoop, you’re effectively negating the effects of both your front intake fans and one of your top exhaust fans.
If you have a smoke pen or stick, you can see this yourself—place it in front of your PC case and watch the cool air coming in from the front go straight out of the top exhaust fan before it has a chance to reach the CPU fan. Two (or more) exhaust fans at the top effectively nullify the benefits of your front intake fans because the air hasn’t had a chance to cool anything.
Noctua pointed out this inefficiency in its airflow guide using the Fractal Design North case, noting that setting the front top fan to intake resulted in the best cooling performance.
Real-world tests confirm Noctua’s theory
Although I trust Noctua more than I trust myself (after all, the brand is famous for its world-class fans), I still wanted to put the theory to the test.
My first instinct was to set all fans to a fixed speed of 60% and run AIDA64 Extreme. However, I quickly realized that my PBO-overclocked Ryzen 7 7700 would simply hit its Tjmax of 95°C and stay there until the test ended.
So instead, I turned to gaming, since video games provide a much more realistic representation of everyday use.
Since my two top Arctic P12 fans are far more powerful than the stock Thermaltake fans at the front of my case, I set the front fans to 100%, limited the CPU cooler to 60% (roughly its average speed during gaming), and set the top Arctic fans to 50% for both tests.
This setup allowed me to keep the top fans at a speed that reflects real-world gaming use, while running the weaker intake fans at full speed to generate enough cool airflow for the test to be meaningful.
I launched Red Dead Redemption 2 and ran the built-in benchmark. Spoiler alert: my results matched Noctua’s findings.
With the traditional configuration of two top exhaust fans, my average CPU (Tctl/Tdie) temperature was 70.0°C.
After flipping the front top fan to intake, my CPU temperature dropped to 68.1°C.
That’s nearly a 2-degree drop from a simple fan orientation change that didn’t cost me anything. For context, you could achieve a similar improvement by upgrading one of your case fans to a more powerful model.
Tech Overwrite saw similar results in his testing, with a 1.5°C drop when running one top fan as intake and one as exhaust. His video also includes clear smoke demonstrations that show what happens in the traditional setup, along with other visuals that make the core issue easier to understand.
When flipping your top fan to intake helps, and when it doesn’t
You’re now probably tempted to flip your front top fan to intake. After all, our tests confirm Noctua’s theory, and it would give your case more positive air pressure to keep dust out, so why not?
For starters, although these findings suggest that flipping the front-most top fan to intake can help with cooling in traditional PC cases, there are some scenarios where this piece of advice doesn’t apply.
For example, if you’re using an AIO to cool your CPU, this is irrelevant. Our main concern was that top exhaust fans were pulling cool air away from the front, but AIOs bypass this issue by moving heat to a radiator, which usually sits at the top or front of the case.
In setups that involve AIOs, you generally want your fans either drawing air in to improve CPU temps or exhausting hot air out of the radiator to optimize overall case airflow. That said, this is a simplification—running your own tests is the best way to see what works for your specific setup.
Another key factor is whether your PC has bottom-mounted fans. In those cases, setting a top fan to intake can create unwanted turbulence. The fans can end up working against each other, with the top intake pushing hot air back toward the bottom instead of out of the case, potentially overheating your graphics card in the process.
The good news is that if you already have bottom intake fans, your front intake fans aren’t as critical, so sticking with a traditional top-exhaust setup usually works just fine.
While traditional PC-building advice is to always set top fans to exhaust, there are plenty of cases where that isn’t actually the best setup. In many standard cases that pull cool air in from the front and exhaust it out the top and back, flipping the first top fan to intake can actually improve CPU cooling.
Ultimately, for the quietest and most effective cooling, go through a variety of different fan configurations to see what works best for your system.
