The Problem of Bottom-Mounted Power
When Intel designed the ATX form factor, the power supply fan was designated as the primary case fan. Being placed at the top of the case, a power supply could easily draw heat away from the hot CPU and VRM components, expelling it out the power supply’s rear panel. That’s also why ATX power supplies are mounted “upside-down”, with the circuit board on top and the lid on the bottom, so that a lid-mounted fan would also be on the bottom, drawing air out of the case and into the power supply.
The design allowed larger fans to be used, and eventually some manufacturers chose fans as large as 135mm. For most systems, this large fan was enough to take care of all the case’s airflow needs, and that’s true even for today’s mainstream parts.
Moving the power supply to the bottom of the case has several negative effects. First, it takes the power supply’ fan away from the top, so that another fan has to be used to remove case heat, adding noise. Second, it makes cable management more difficult, often times with the ATX12V cable not reaching its motherboard connector. Third, as the power supply casing warms up, it heats the graphics cards.
The only benefit of putting a power supply at the bottom of a case is that the power supply itself runs cooler temporarily, drawing dusty air from under the case until it plugs up. Once the power supply is full of dust, carpet/paper fibers and whatever other small items clutter your desk or floor, the benefit of having its own cooling path disappears.
We excused the power supply location on our high-end build because the change was needed to put the radiator mount on top, accepting the sacrifices needed to achieve our liquid cooling aspirations. But the Centurion 590 doesn’t have enough space above the motherboard to mount a radiator in addition to fans, and our $1000 system uses air cooling anyway.
Unwilling to accept any information pointing to the inferiority of bottom-mounted power supplies, many readers have previously asked questions to invalidate this argument. The most common of these is “If there are so few advantages and so many disadvantages, why have so many companies made the change?” The answer is easy: Because buyers ask them to. Companies will produce whatever customers want, and the reason it took some so long to implement new designs applied long ago by competitors is that most manufacturers chose to wait out the trend, to see if it would stay around, not wanted to produce a design that would be rejected by a change in trends.
Another common question is “Well then, why does BTX have the power supply at the bottom”. The answer is that it doesn’t, at least not by Intel’s design. Some manufacturers have made the switch in their BTX cases for the same reason as above, that is, customer demand.