Many modern cases still have a top-mounted PSU. I was wondering: why? What's the disadvantage of bottom-mounted PSUs?

The original ATX Chassis was designed so that the PS fan would cool the CPU heat sink via a duct and many systems with low wattage CPU's are still built this way.


You will have to select a more expensive PS with longer cables.

I know, but even cases like the new Solo II have a PSU on top.

I prefer PSU on the bottom, but new cases do have them on top like the Antec Solo II and Silverstone TJ08-E with top vents, on the solo II i can see a gain with the fans more align with the mobo components (GPU/CPU), on the TJ08-E uses a single frontal fan but also with the PSU on top it blow to hole mobo.

The solo II has a vent above the PSU, so you can have it draw air from the outside - as such, it should be functionally equivalent to most cases with bottom-mounted PSUs (and it should be a particularly good choice for a fanless PSU). It's definitely not the same as traditional ATX cases where the PSU draws air from the inside of the case.

Ah. But then PSU fan noise has a direct exit path.

My system has no direct noise paths to my ears because the top fan position is blocked and the air inlet area on the front is double baffled.

I use only one fan in the upper chamber of my P180 case. The top fan position is blocked and and the vents at the expansion slot area are blocked. Air enters through the filter on the front and exits through the rear fan position. A 3" long ducted fan assembly is constructed from a Scythe 120mm PWM fan sandwiched between two 120mm fans with the guts cut out and is mounted on the rear chassis fan location. This ducted fan assembly aligns well enough for adequate air to be drawn through the Scythe NINJA CPU cooler. The fan is connected to the CPU fan connector and variable fan speed is enabled. The fan runs at around 650 rpm at idle and 1500 rpm when re-coding a video.

At idle this computer is silent during the day and when things quiet down in the late evening I can here a very gentle, faint 'whoosh'.

-------My Main System----------------------------------------------------------------------------------------------------------------------------------------
Antec P180 spcr_edition Case, Athlon II X4 640, Scythe NINJA, ASUS M2A-VM HDMI, OCZ Solid 3 120 GByte SSD, 500 GB FXD, WinXP

Not only that but any spills, etc go straight into the PSU

I very much dislike top fan vents for this reason and wish it was easier to find a case without them, as I'm only going to have to try and block them up to protect my valuable components from damage and it's not easy to make them water-tight. I've always been quite happy with top-mounted PSU's, with the fan facing down, but I'm OK with bottom-mounted ones as long as there's an external removable filter, so that the PSU doesn't get full of dust and I can easily take the filter off to clean it from time to time.

IMO, I don't think there's any reason to move the PSU to the bottom location apart from marketing. It has just become a hype. Thermal gains are negligible. The "convection theory" falls apart if there are any fans in the case.

In 1998 you had generic beige boxes with moulded fascia.
In 2004 you had "Prescot ready cases" and windows.
In 2006 you had aluminum cases and kitsch cold-cathodes.
In 2010 you had bottom-mounted PSUs, black painted interiors and motherboard cutouts.

All of the above don't improve much performance or thermal-wise, but are simple selling-points to convince you that you need to change a perfectly good case to a newer, allegedly "improved" one.

IMO they`re far from negligible, especially when dealing with higher ambient temps and slower psu fans. Moving the psu at the bottom noticeably reduced exhaust temperature for me and allowed me to use a slower fan. Another drawback with the typical placement at the top is that the psu fan is often competing for airflow against the rear case exhaust. Having the psu at the bottom with it`s own intake you don`t have to worry about it messing with case airflow.

Now, in a low power system the above could become a non issue but in a machine producing a fair amount of heat having the psu at the bottom can produce some tangible benefits.



I never liked top vents, especially after a friend of mine brought in his pc a few days ago, after having spilled coke into it..

I have a different experience. My previous system (C2D 4300 OC'ed to 3GHz) has been delegated to serve as my parents' general computer. I've moved the system to a new case with the PSU at the bottom. Temps are exactly the same as they were while the PC was housed in my Antec Solo. The heatsink is a passive Scythe Ninja 2.

Current system (see my sig), despite a substantial overclock, remains cool at all times, thanks to the Solo's design, i.e. optimal front-to-back airflow.

I don't think fans running below 1000RPM will ever compete for airflow. There's simply not enough suction.

A C2D isn't exactly a high-end CPU that produces a lot of heat. I assume you didn't have a dedicated GPU either.

Again, a 5750 isn't a high-end GPU...
You can't just generalize from your system.

How about the temps inside the psu though? Even if the psu fan stays at a low speed, it doesn`t mean that all the components inside are adequately cooled, since there are no sensors on each one of them. While the main heatsink could stay cool enough, components receiving less airflow like the secondary capacitors could suffer. At best you`d get reduced efficiency because of the increased temperatures.

Well, it isn't a high-end CPU but it does produce a lot of heat. Please consider that there's a heavy overclock involved - from 1.8 to 3.0GHz and the production process is 65nm. All energy-saving features had to be turned off and voltage has been increased for stability reasons. All in all, it is a hot CPU, but nothing Ninja 2 couldn't handle passively.
And yes, I had a dedicated GPU - first a 8800GTS 320 (G80), which was VERY hot and inefficient. I swapped it for HD 4850, which is also a hot card. This one is still serving its duties, as it's cheaper and greener to keep a separate card and pay for the electricity than to buy a new MOBO just for integrated video.



Granted, it isn't high-end, but it's passive and all the heat is dumped inside the case. Temps are 40 at idle and 80 under load. I'm quite certain that a hotter card would do just fine, I was hoping to swap it for a passive HD6770, but will probably wait for the 7xxx generation to arrive (5750 is adequate for my gaming needs just now).



The savings on the increased efficiency of a cool PSU will take ages to equal (or justify) the cost of a new case (that basically was my point). A test should be conducted - same components, same fans&flow - different cases (one top-mounted PSU, one bottom-mounted PSU). And then measure the temperature of the air coming out from the PSU.
Perhaps this has been done somewhere? Does anyone have a link?

What's a lot of heat?
It's still just a dual core. Others might have quad (or more) cores, producing even more heat. A friend of mine has a Q6600 overclocked to 3+ ghz.
A 8800 GTS uses less energy and produces less heat than more modern cards (AFAIK).
A PSU that's fed cold air takes less to cool than a PSU that's fed hot(ter) air.
AFAIK there's no reason why mounting a PSU at the bottom would require a larger case.

This CPU produces 135W of heat. (when overclocked, it's TDP is 135W to be precise) That's a lot
It doesn't matter how many cores you have - only the amount of heat generated (or, speaking precisely, the TDP). A single core can produce that much (remember Pentium4?), as well as a hexacore. The efficiency is the key and it varies.



It consumed 108W of energy (peak 3D) but produced quite a lot of heat due to its inefficiency. It also idled quite high. On the other hand, the heatsink was designed in such a way that it blew hot air outside the case. A passive card can't do that.



I'm not going ot deny that



I didn't say that.

I totally agree.

I have seen builds with original design cases where the case fan was so much more powerful than the PS fan the there was no air flowing through the PS.

I know. Where did you get that 135 W value from?

You do know that every W that enters your computer as power is transformed into a W that exits your computer as heat, right?
Still, 108 W isn't much for a videocard.

You know, every time you dismiss something fairly high-powered of his, you're further restricting the cases where a bottom-mounted PSU is relevant. Most machines now are in the vicinity of 100w total, being Intel systems with integrated graphics and a couple drives. Many will be more realistically in the vicinity of 50w. If his ~250w machine "isn't very much", then what does need a bottom-mounted PSU: SLI rigs with Extreme or Bulldozer CPUs? That makes the orientation irrelevant to the vast majority of people.

The vast majority of people don't care about noise that much, so it's already irrelevant for them.
BD/CF/SLI are exaggerated, I was thinking about R6950 or GTX 560 Ti...

That said, I'm still not aware of any disadvantages of PSU on bottom.

I'm glad it's not just me then. If you know of any decent budget (<£40) cases without them (I need four 5.25" bays (in fact preferably all the way from top to bottom as then I can fit my HDs with adapters, but if not either a removable drive cage or one with half-decent vibration-dampening rails) and external removable intake fan filter(s) or space to fit magnetic ones) please let me know, as everything I've found seems to have top vents. I was looking for decent cable management, but I'll sacrifice that now in desperation, and I couldn't really care where the PSU is (providing if it's bottom mounted it has an external, removable filter).

If the ambient temperature is high enough, the position of the psu can make a difference even in systems with a moderate power draw. Besides, the same can be said for most parts talked about over here: most pcs don`t need a thermalright hr-02 cooler for example, yet most of us are glad such things exist.

In practice, mounting the psu at the bottom has allowed my 150 watt pc to remain inaudible at load even in summer and at the same time considerably reduced the temperature of the psu casing. Granted, not everyone has to deal with ambient temperatures in excess of 35 degrees c, but how about people with 250 watt pcs?



The point I wanted to make was that when building a new machine, of from the manufacturer`s/designer's POV, when developing a new case, moving the psu to the bottom is a sound choice and not just a marketing gimmick. It adds no extra cost or weight and at worst offers a small improvement in temperatures and noise. The main drawback is that a longer 4pin power cable is needed something that could be a bit of a PITA before psu makers adapted, but nowdays, it`s much less of an issue.

I think I've found a suitable case at last (AeroCool VS4).

It's not perfect and I've posted about the pros and cons in my thread here but it looks like the best available compromise.

I'm surprised no one has mentioned the fact that the optical drive bays are typically placed at the top front of the case. Putting the PSU at the top makes use of the dead space behind these optical bays and allows a more direct front-to-back airflow.

Add me to the camp of people who aren't entirely convinced that having the PSU at the bottom makes that much a difference (generally speaking) in temperatures. The most often invoked reason I hear in support of a bottom-mounted PSU is that it doesn't compete with the cpu exhaust fan or get affected by cpu heat. Except I have a hard time seeing how this isn't also a problem in "extreme gamer" systems with double/triple SLI configurations. My housemate has a Tri-SLi system and his graphics cards are all stacked on top of each other right on top of his PSU. His third graphics card essentially sits on top of the PSU intake fan.

You'd put the PSU intake at the bottom, not at the top.

I think the only time a PSU with an separate intake is useful, if for a system with very poor exhaust, where the PSU would have to exhaust more heat than expected. In this case, allowing the PSU to get fresh air from the outside would certainly help with the noise. For any other well built system, I don't think it makes that much of a difference...
My system in the old solo pulls 330 W from the wall at load (OCed Q9550 and dual OCed GPU) and the exhausts are a 120mm Nexus at 9V and the fan in the X650 (can't hear it but it's running). So yeah, I can hear the system when I sit right next to it on the carpet, but if I'm more than a couple of feet away, I cannot hear anything.

Your first point is a cop-out: I don't believe the bottom-mounted PSU makes any difference to noise for the vast majority of builds, so people's preferences don't matter. Anyone building even a typical gaming machine in a mid-tower should be able to have it quiet and ventilated with either orientation, and only people who care enough to build their own machines will even be aware of the choice of PSU location, so we're starting with a more aware group of users.

That said, I wouldn't base my choice of case on the position of the PSU. It does make cable management harder/ more annoying, but many cases do that on their own. It's neutral, to me, I just don't believe it's the big deal some are making it out to be for the vast majority of builds.

It does influence me somewhat, as I don't want my PSU sucking unfiltered dust from outside the case so if it doesn't have an external removable filter it puts me off. I don't particularly want to put the PSU fan side up either, as that not only moves the cables further from where they need to go (which might mean having to buy/use extension cables) but also means that screws, etc can be dropped in it when working on it!

I'm buying a case which is going to have a bottom mounted, fan side up PSU (I'll either fit a filter on top, or just cover it when working in the case to stop stuff falling in), simply because I can't find anything else that comes close to what I want , but if I had a choice between a top mounted PSU and a bottom mounted one with external, removable filter I don't think it would be important, although it does tend to be easier to reach the PSU's power switch when it's at the top of the case, as you can just lean over, or if it's under a shelf slide your hand along the top, whereas to get to it when the PSU is bottom-mounted you need to have space around the case.

Then the PSU's thermal effect on the rest of the system is really no different from having the PSU on top with the intake at the top of the case. Then PSU placement really comes down to preference, as there's no objective advantage of either position over the other.