How to keep dust out of my power supply?

Thanks for the input, guys! I've positioned the PSU this way, because that's what the manufacturer (Seasonic) recommends in the PSU's manual, at least when using the PSU in this "hybrid mode" (so normally it cools passively and only with high temperatures the fan will rotate). So even in this particular case (hybrid mode (and thus most of the time passive cooling) + PSU at bottom of case) you would position the venting/fan side downside, even when Seasonic says otherwise?

I've struggled with this myself to be honest: on one side I would argue that hot air should be able to leave the PSU (and case!) as soon as possible (!) and fresh air should be able to enter the PSU, but on the other side I also understand that hot air rises and thus more easily leaves the PSU-in-a-passive-mode upwards than downwards (although it passes the rest of the case...). Also, even if hot air from the PSU enters the rest of the case this way, IMHO this doesn't necessarily mean there's an issue if the rest of the air flow is good enough (except when you're trying to lower temps just for sports ). On the other hand one might also say that you could reverse the PSU and make the same reasoning: if temps are not a real issue for the PSU it's not THAT important and you could reposition the PSU as well.

Then there is another final aspect: my PSU runs in hybrid mode. It runs passively till a certain temperature, then it switches to active cooling (through the built-in fan). With active cooling it's certainly the most logical to position the venting/fan side downside. Even IF this would not be the best way for passive cooling it's not a big deal because it will switch to active cooling on time. The only drawback could be that with the fan side downside I could perhaps run the PSU longer in a passive way (before it switches to active cooling): that means more energy and more noise (but I'm a little pedantic right now ;-)).

Bottom line: I'm not 100% sure what's the best positioning if a PSU is using passive cooling ALL the time. Disadvantages and advantages, you know, but the question is which are dominant...
Also, if using hybrid mode like me and there is a choice to run fanlessly as long as possible (for whatever reason), the question is basically the same. But if there's no real reason for this, perhaps it's indeed true it's better to put the fan side downwards - the worse thing that can happen is that active cooling would turn on more rapidly (and that's ONLY if this positioning would not be ideal for passive cooling!).

Greetz,
Pedro

@madmatt30: Thanks for your reploy. And I'm sorry, I see just now that you've written earlier "Psu fans take air into the psu not blow it out !". That makes a huge difference, as I thought they did the opposite! And yes, I just took a look and you're right: the fan is intake (towards the PSU)! Anyway, that doesn't change anything related to passive cooling (as then there is no fan running), but it does influence the stance on how to position the CPU related to active cooling (and thus also to hybrid mode): when active cooling is enabled and the fan side is faced upwards, that means it doesn't blow out the PSU's hot air, no, at the contrary, it even attracts more hot air from the rest of the case!

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This is what Seasonic has written in their PRIME manual:
2.1 Position your PSU
Top of the case: bla bla bla
...
Bottom of the case:
For PRIME: The fan control setting determines the optimal PSU placement.

In Hybrid mode: Seasonic recommends to install your PSU with ventilation top cover facing UPWARD, towards the direction of the motherboard. This will ensure natural heat dissipation (Figure B).

In normal fan control mode: The direction of mounting is optional. However, it is not recommended to install the PSU with the ventialation top cover facing DOWNWARD if there are no ventilation holes on the bottom of the case (Figure C).
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So... Seasonic thinks that in the passive scenario facing the ventilation top cover upward should have the best results (for the PSU only or for the case as a whole...???). So passive heat dissipation through the rear AND the top (even tough it passes the mobo/CPU/...) should be better than only using the rear (and a little bit perhaps the bottom, as that won't do a lot, as hot air rises). Well, that seem to be what Seasonic thinks and perhaps they only talk about what's best for the PSU and not for the system as a whole.

They also think that in the active scenario it doesn't really matter. Facing the ventilation top cover downward speaks for itself: cool air from the bottom is sucked up by the fan and hot air leaves the PSU at the rear side, partly because of the air flow caused by the fan; the rest of the system is not influenced. Sounds great.
Considering facing the ventilation top cover upward as an equivalent solution though is a bit weird, isn't it? The aspect of "hot air rised" (natural heat dissipation) is almost nihil here, as the fan sucks air into the PSU. And that air is hotter than air from the bottom (and even if sucking air downward would be theoretically better, that difference must be very minimal), although that air still comes from the lowest third of the case (so it shouldn't be THAT hot). I can accept this as a good solution only when there are no holes at the bottom of the case, but then they should have phrased their text better!

Anyway, when Seasonic thinks passive and active cooling is great when facing the ventilation top cover upward, then I also understand why they think this is the placement for hybrid mode.

Personally I would definitely face the ventilation top cover downward in an active-only scenario. On the other hand I would probably (am less sure here) face it upward in an passive-only scenario (except when temperatures would be too high in my system, then I would try to face it downward too).
For hybrid mode, well, that's a difficult one... It depends on how long you definitely want to stay in the passive cooling range (for whatever reason like noise reduction for example), the temperature of your system, the percentage of time you expect to run passively, etc.

I think I'm gonna try it facing the venting side upwards (so following Seasonic's recommendations), reasoning that I start without the fan running and that it will stay that way for a while as long as I don't really stress my machine (which should be the largest part of the time - I do want to do some heavy duty tasks, but most of the time I'm developing, running virtual machines and doing "boring" IT research, so I'm not the type overloading my machine every day for a few hours ). When I do want to do some more hard work (for the machine, that is) the fan will probably start to rotate as temps rise. In that case I'm pulling some relatively warm air into my PSU, but perhaps that's still ok enough.
Or... I could play more on safe by facing it downward, so I always have the best conditions when my system gets stressed and gets warm. Only drawback: as long as that's not the case and the fan doesn't rotate (probably the largest part of the time) I'm not following the recommandations for passive cooling, thinks will be a little warmer and active cooling will start a bit earlier.
Aaaaarrrgglll...

Perhaps, and I say perhaps, Seasonic thinks/knows the PSU will run most of the time with passive cooling AND active cooling when facing upwards isn't as bad as it sounds (although, yes, perhaps a bit worse than downward) - perhaps those PSUs are designed to handle this decently? Perhaps taking care of the PSU in passive cooling mode is more important than taking care of them in active cooling due to the way it is designed? Perhaps the fan attracts not-so-cold-air-anymore in active mode, but at a point where it's still better than not running and dissipate it very well through the rear side?

One special advantage of facing it downward is that dust cannot enter the PSU that easy, although it's possible to put a 140mm dust filter (SilverStone for example) on top of it (yes yes, it impedes air flow a bit, but as long as temps stay okay, that's not a real issue and you know, dust doesn't do any good for temps either).

A lot of text, but I think this is one of the most difficult choices I've ever had to make (well, at least concerning building a PC ).

More feedback is highly appreciated

Pedro

"...and one that takes all of 3 minutes to change the orientation of the PSU."

Changing the orientation is not a big deal (except perhaps a little bit for cable troubles, especially when all cables are bound together with cable binders), but when talking about hybrid mode different scenario's should be tested to see what fits best. It's not a disaster to do so, but still...

BTW, one reason I can think of why Seasonic says the orientation in an Always-active-cooling-mode doesn't matter, is perhaps that when the fan side is faced downward, it's more difficult to suck a lot of air (because of the small gap between the bottom of the case and the floor/desk/...). Perhaps their experience learned them that facing the fan side upward is not worse (or not much), even though it sucks air that is hotter, just because it can suck up a bit more easily compared to the reverse orientation.

This topic is not as black or white as I first thought. Weighing in every argument and reasoning I think the best choice to start with in my situation (@bottom, hybrid mode, my air flow, my case, my components, my usage, etc.) is the recommended one by Seasonic, but I must agree that there's a high chance that the other solution would be better. I'm gonna monitor the whole thing for a while (in different scenarios) and perhaps I'm gonna try it the other way around then for a while, just to see what's best (IF one is better than the other).

Thanks for all the input, guys! Sometimes it's needed to make you think broader, even though there is no clear answer.

Greetz,
Pedro