Here's where I deviate from accepted "convention" (as I do in so many ways... bwahahahahahah). This post might get a few PSU makers and sellers ticked, but what the hey... mine is only one voice in the wilderness.ColdFlame (from another thread...) wrote:I actually prefer the fan at the bottom of the PSU because it sucks hot air from the CPU.
The whole dual fan PSU idea seems to have been promoted by Enermax & AMD around the time the original Athlon appeared. [Some of may recall how the Athlon pushed the then-standard 250W PSUs (especially no-name generic ones) to overload, causing all kinds of mysterious instability issues that were eventually traced to inadequate PSUs.]
Anyway. Back then there was not much thought given to heat and airflow in PCs. If you think cases are airflow impeded now, you should take a look at the old ones. They were worse. So with the extra heat from the Athlon, the extra fan in the PSU sucking air from the CPU area made a lot of sense, it got rid of the heat that the systems weren't handling well otherwise.
Now you know those Athlons were hot in their day, but early ones didn't exceed ~35W. The extra heat didn't really change the PSU operation much, especially as the industry was not paying much attention to noise. (Of course, later the T-birds reached ~70W...) For the last couple years, AMD has even had a diagram in their popular system assembly guides showing this blow-thru-PSU heat exhaust as the preferred case cooling.
Skip ahead to the present.
You now have CPUs pumping >80W of heat. For quiet enthusiasts, PSUs have thermal control to keeps its fan(s) running slow except when the internals reache a high temperature. PSUs of 400W rating are commonplace, as are VGA cards that dissipate >60W. Dual HDDs are almost ordinary. Yes, PCs in North Am seem to have followed the auto industry lead -- big gas guzzlers are the norm.
So when a fan sucks the rising heat from the CPU (and everything else below) and blows it through the PSU, what do you figure happens?
Right -- the PSU & that controlling thermistor gets HOT. This then leads the PSU fan control circuit to... speed the fan up! Which leads to more... NOISE.
So depending on the particulars of the PSU fan controller, your PSU fan could be going up & down in speed and noise like a yoyo. To me, this is much worse than a steady noise.
With CPUs and systems pumping out as much heat as they do, the idea of of using the bottom mounted fan in a PSU to suck more hot air out only makes sense if you...
1) don't care about how noisy the PSU fan gets as it speeds up in response to the extra heat, and
2) don't care what the extra 50-150W of heat going through the PSU will do to its longevity & stability.
3) have a well ventilated, modest-heat system that does not cause the PSU fan to speed up.
This is why I think the bottom mounted fan PSUs need to be used very carefully if you want to have a quiet system. No question, if the overall system heat is modest, it can still work fine. I'm thinking something like max 125W DC power draw. Maybe less. It depends how quiet is quiet enough for you.
My ideal case config for higher power systems is a straight-through airflow path for the PSU (with a duct from emty top CD bay to PSU intake side) with the PSU fan at the absolute minimum. Yes, this means you need to have good heat exhaust around the CPU area, but you need this anyway, and 2 quiet slow fans are almost always quieter than one fast noisier one.
This is not new, it's the idea Lilla ran with in Building a PSU channel but first explored here by early contributor Leo / powergyoza in his Quiet Dualie article
OK, let's hear the counterpints. uh... I mean points. I wonder if any PSU engineers will say anything?