Fan at top or bottom of PSU box.

[alex2007]

I'm buying a new PSU and am confused as to the location of the larger fans in the PSU. Some appear to be at the top, where airflow is often restricted or non-existent; others at the bottom, which seems to me the obvious position. None of the manufacturers' sites address this issue.

Are there any relevant standards, does a top-mounted fan require a specific case with an airflow channel above the PSU, or are the PSUs simply reversible and can be used in either orientation?

Thanks for all replies.

 

[Zap]

Hello alex2007, and welcome to AnandTech Forums.

A power supply can be oriented in any direction as long as nothing blocks the fan. If you see pictures with the fan on "top" or on "bottom" that's just the way the picture was taken.

Generally speaking if the ATX case has the power supply on top, it would be mounted with the fan facing down into the case as obviously if the fan was facing upwards it would be blocked by the top of the case.

With an ATX case that mounts the power supply on the bottom, some cases have bottom vents so you can mount the power supply whichever way you want. Other cases (example is original Antec Nine Hundred) do not have bottom vents so you must mount the power supply with the fan facing up.

 

[alex2007]

Thanks for the prompt reply.

> A power supply can be oriented in any direction as long as nothing blocks the fan. If you see pictures with the fan on "top" or on "bottom" that's just the way the picture was taken.

I note that the mounting holes are asymetric - the bottom left mounting screw is inset about half an inch. If the manufacturers intend either orientation, do they provide duplicate mounting holes to suit?

 

[Zap]

Cases with PSUs on top usually have the mounting holes such that the PSU fan faces down. However, I've seen plenty of cases where you can mount it either way, either having two sets of mounting holes or using a mounting bracket (such as what Lian Li does).

 

[Modelworks]

I think a lot of the confusion was caused by the idea that heat rises so when towers were first produced they placed the PSU at the top. The reality though is that when designing cooling the concept of heat rising only applies when you have no forced air flow. Once you start adding fans to the case you can make the heated air go anywhere you want.

 

[Meghan54]

I think a lot of the confusion was caused by the idea that heat rises so when towers were first produced they placed the PSU at the top. The reality though is that when designing cooling the concept of heat rising only applies when you have no forced air flow. Once you start adding fans to the case you can make the heated air go anywhere you want.

And people still are confused by that concept. True, heat rises except when faced with 35cfm of air movement pulling it out the rear of the case.

 

[killster1]

Hello alex2007, and welcome to AnandTech Forums.

A power supply can be oriented in any direction as long as nothing blocks the fan. If you see pictures with the fan on "top" or on "bottom" that's just the way the picture was taken.

Generally speaking if the ATX case has the power supply on top, it would be mounted with the fan facing down into the case as obviously if the fan was facing upwards it would be blocked by the top of the case.

With an ATX case that mounts the power supply on the bottom, some cases have bottom vents so you can mount the power supply whichever way you want. Other cases (example is original Antec Nine Hundred) do not have bottom vents so you must mount the power supply with the fan facing up.

YA correct put them which ever way you want but i suggest putting the fan on the bottom always, i have antec 1200 and the psu goes on bottom with fan on bottom, (has like 1" clearance from the bottom of the case and no bottom air intake and works perfect like that and runs cool. no idea about the antec 900 tho.

heat does rise and fan pumps the air through more components if you put it on the bottom. Just my opinion have done zero tests just going by logic.

 

[JTsyo]

Are PSU fans that point into the case, outlet fans? I figured it sucks heat from the inside, though the PSU and out the back of the PSU, outside the case.

 

[alex2007]

Thanks for all replies - now have a clear idea of what's what.

I remember reading way back about a 'new' BTX case that never seems to have caught on, in which the PSU was at the bottom of the case as per the above suggestion, and seemed to incorporate better heat-flow engineering principles. Does anyone else recall this?

 

[fffblackmage]

Thanks for all replies - now have a clear idea of what's what.

I remember reading way back about a 'new' BTX case that never seems to have caught on, in which the PSU was at the bottom of the case as per the above suggestion, and seemed to incorporate better heat-flow engineering principles. Does anyone else recall this?

A lot of ATX cases have the PSU mounted at the bottom. It really doesn't matter where it goes.

I think the he primary benefit of BTX was moving the CPU closer to the front of the case where it gets fresh cool air first. I believe Dell still uses it?

 

[Viperoni]

Are PSU fans that point into the case, outlet fans? I figured it sucks heat from the inside, though the PSU and out the back of the PSU, outside the case.

A PSU will (almost) always exhaust air out of the case, so yes, that does make it an exhaust fan of sorts.

 

[jsalpha2]

I still prefer having a fan on the back to blow air out of the case. If the power supply is mounted upside down it would be blowing down into the case and pulling air in through the back, unless you reversed the fan. In order to reverse the fan so it would pull hot air from the CPU and blow it out the back, you would almost certainly void the warranty. But if the fan is on the top and there is little or no room between the power supply and the top of the case there would not be much air movement, so that would be the worst way to mount it.
Even if you only use 3 screws, because of the holes not lining up. So for most non gaming builds I like the Antec EA500 or something shaped like it.

 

[ElFenix]

Are PSU fans that point into the case, outlet fans? I figured it sucks heat from the inside, though the PSU and out the back of the PSU, outside the case.

ya, PSU fans are outlet fans usually. way back when ATX first got started there usually weren't other exhaust fans in a computer. so the PSU was at the top to get rid of the heat all the other components generated.

i do remember there being some dispute at one point whether a power supply fan should blow in or out. i think it was when 120mm fans first became common. the argument was that big fan could blow air from outside (though warmed by the PSU) down onto the CPU cooler (which back then usually featured a 60 mm fan; 80mm fans were just coming around then), and help cool the CPU.