# Power Supply fan Placement?



## okrobie (Aug 27, 2011)

I notice that all of the newer PSU's have a 140mm fan on the top of the box. I'm concerned about buying one because it appears to me that in my case, the fan will be blowing directly into the "roof" of the case. Surely I'm missing something about the placement of these power supplies because they no longer offer the type I am replacing in the high end units. The PSU I am replacing has 80mm fans at either end of the PSU box which blows directly out of the rear. Can someone please straighten me out.


----------



## Niram (Jun 20, 2009)

You just flip it, all good power supplies have 2 fans, one in the case that blows down and the other on the rear blowing out


----------



## Jay_JWLH (Apr 30, 2008)

It should pretty much work out like this:


----------



## gcavan (Aug 13, 2009)

> You just flip it, all good power supplies have 2 fans, one in the case that blows down and the other on the rear blowing out


Not right.

All supplies will have either:**
A rear mounted (usually 8 or 9 cm) exhaust fan, or;
A bottom mounted (12-14 cm) intake fan which draws air in and pushes it out the rear.

In computer cases which allow for a bottom mounted supply, the power supply draws outside air through the bottom of the case (usually through a filtered vent). When the supply is top mounted it draws air from inside the case itself.

** PS: Of course there are also a few fanless supplies.


----------



## Tyree (May 10, 2009)

If your case uses the bottom PSU mount and the PSU has one large fan you can turn the PSU upside down to blow into the case. If the case has an adequate opening and enough clearance under the case, you can mount the PSU with the fan blowing down.


----------



## okrobie (Aug 27, 2011)

Thanks all for your replies. Once I got one in my hands I saw how dumb my question was. But... as they say, live and learn.


----------



## Johnny1982 (Oct 15, 2010)

I agree with Gcavan on this. My Corsair HX750w has a single 140mm fan taking cold air in from the bottom of my case (bottom mounted PSU design) and exhausting it out the back. My dad made me a box with a chamber for the PSU to suck in cool air from the bottom and not directly from the ground and the case also has a dust filter on it, which I clean periodically.


----------



## Tyree (May 10, 2009)

Either way is acceptable as long as the PSU can get an adequate amount of air coming in. High quality PSU cooling fans turn at low RPM so there's not much concern where the air is discharged.


----------



## Bill_Bright (Dec 17, 2004)

> all good power supplies have 2 fans


As noted, that is definitely not true. 

They key thing is the PSU fan or fans should draw air into the PSU, then exhaust the hot air out the back of the computer case. A PSU should never exhaust hot air back into the case. 



> High quality PSU cooling fans turn at low RPM


Actually, quality has nothing to do with it. The advantage of large cooling fans (120mm or larger) is they move massive amounts of air while turning at a lower RPM (as compared to smaller 80mm fans - which must spin much faster to move the same amount of air). Besides the greater movement of air, the lower RPM means means less noise.

There is no relation between the "quality" of a fan and the "size" of a fan. Quality comes from the engineering that went into to designing the aeronautical characteristics of the fan blades (pitch, rake, angle of attack, etc.) using the same aeronautical principles as airplane propellers and wings. The manufacturing techniques and the precision and type of fan motor bearings used also greatly influance "quality". For example, precision ball bearings and fluid bearings are generally used in quality fans while sleeve bearings may be used in cheaper fans - again, however, totally unrelated to fan size.


----------



## Tyree (May 10, 2009)

Good quality PSU's generate less heat than lower quality units so the cooling fan turns at lower RPM's. My SeaSonic & XFX units turn so slow they're barely audible even under a heavy load.
With bottom mount cases, I commonly mount them with the fan blowing into the case without any heat issues. The rear 120mm fan picks up the barely warm air and draws it out.
Again, either mounting position is acceptable.


----------



## Bill_Bright (Dec 17, 2004)

> With bottom mount cases, I commonly mount them with the fan blowing into the case without any heat issues.


So you crack open the PSU case and turn the fan around? I advise against that. PSU makers generally know what they are doing - at least the better makers and you really don't want to pump heat into the case interior. So I disagree that either position is acceptable. You generally want front to back air flow through the case and heat naturally rises so it is best to accommodate that, not counter it. Turning the fan around counters both those principles. 

And also, PSUs are generally considered to not have any "user serviceable parts" inside. This means you might void the warranty if you open a PSU case. Not to mention, anything that plugs into the wall can kill - so certainly, there are precautions to take.



> Good quality PSU's generate less heat than lower quality units so the cooling fan turns at lower RPM's.


While it is true, better quality PSUs tend to be more efficient - thus generating less heat, resulting in the fan being able to run cooler, but again - quality, is not really the determining factor. You can have a cheap 750W PSU powering a lessor computer that only needs a couple hundred watts max. The cheap PSU would never be taxed so even if inefficient, the fan will not need to spin at full (loudest) speeds.


----------



## Tyree (May 10, 2009)

Apparently my post was unclear. :4-dontkno
I would see no reason to change the orientation of the fan and I would never suggest opening a PSU.
When using a single fan PSU, I simply turn the PSU upside down so the fan is on top as shown below. Higher Quality PSU's use higher quality parts and generate less heat resulting in lower RPM fan speeds.


----------



## Johnny1982 (Oct 15, 2010)

I too was taught at college never ever open a PSU. There are capacitors that still hold charge and can be very dangerous and possibly result in death if handled incorrectly.


----------



## Bill_Bright (Dec 17, 2004)

> Apparently my post was unclear.
> I would see no reason to change the orientation of the fan and I would never suggest opening a PSU.
> When using a single fan PSU, I simply turn the PSU upside down so the fan is on top as shown below.


Ah!  That makes perfect sense and I totally agree. Thanks for clarifying. 

But I still don't agree with,


> Higher Quality PSU's use higher quality parts and generate less heat resulting in lower RPM fan speeds.


I agree totally with the theory, but not really in practice - not as a general statement. You need a qualifier - such as, "higher quality PSUs, _compared to lessor quality of the same wattage_ generate less heat resulting in lower RPM speeds." Bigger PSUs have, among other things, bigger heat sinks that radiate more heat by simple convection. So, as I noted above, a big fat cheap PSU will loaf along and not need to run the fan full speed, while a high quality PSU of lower wattage running at near capacity will have to run the fan at higher speeds.



> I too was taught at college never ever open a PSU. There are capacitors that still hold charge and can be very dangerous and possibly result in death if handled incorrectly.


That should NEVER happen - but sadly, this is another case where theory and real-world don't jive. All ATX and UL approved power supplies MUST have bleeder circuits to quickly drain off any stored charges to avoid exactly what you speak of. But until Man can create perfection 100% of the time, even the best makers can end up putting a faulty or flawed component in the circuit. And who knows what parts the cheap, no-name generic PSUs made in some obscure factory in some 3rd world country are using? Most likely parts made from similar quality raw materials in a sister factory upriver.


----------



## Tyree (May 10, 2009)

Bill_Bright said:


> But I still don't agree with, I agree totally with the theory, but not really in practice - not as a general statement. You need a qualifier - such as, "higher quality PSUs, _compared to lessor quality of the same wattage_ generate less heat resulting in lower RPM speeds." Bigger PSUs have, among other things, bigger heat sinks that radiate more heat by simple convection. So, as I noted above, a big fat cheap PSU will loaf along and not need to run the fan full speed, while a high quality PSU of lower wattage running at near capacity will have to run the fan at higher speeds.


It's not really rocket science. :smile:
To clarify yet again, when speaking of PSU's I only refer to good quality units. I do not use, sell, or recommend any "big fat cheap PSU's".
Good quality electronic components are more efficient. Efficiency reduces stress resulting in lowers temps. Lower temps reduce the demand for cooling resulting in lower RPM's from the fan.


----------



## Bill_Bright (Dec 17, 2004)

Totally on the same page now. It is my normal recommendation to make the PSU a top priority for the very reasons you state - and the last purchase decision of the build to ensure the right size. But sadly, not everyone seeks that advice until after their first build has problems.


----------



## Johnny1982 (Oct 15, 2010)

Looks like we've finally come to a consensus then.


----------



## Bill_Bright (Dec 17, 2004)

> Looks like we've finally come to a consensus then.


And that is, "don't compromise to save a few dollars when buying a PSU". 

I like to use the analogy of spending your hard-earned money on a new Porsche. Would then use cheap, off-brand fuel from the corner Acme Tobacco Hut and Laundromat? You might, but would you expect it to run right? A car engine can miss a beat and keep on running - not so with high-speed digital electronics!

Even so, I also always recommend all computers be on a "good" UPS with AVR too - as yet another thunderstorm rolls through my area and the dog is wrapped around my legs.


----------



## Tyree (May 10, 2009)

Johnny1982 said:


> Looks like we've finally come to a consensus then.


:smile:


----------

