I think it is a common misconception that a bottom mounted PSU contributes to heat in a case. This is simply not the case. As a matter of fact, I firmly believe the opposite to be true.
Let's look at the pros of a bottom mounted PSU with most modern-designed computer cases:
PROS:
-Easier cable management
-Better balancing of the case (less top-heavy)
-Not in the way of optical drives
-Less obtrusive
Now let's look at the alleged pros of a top mounted PSU:
-Less prone to contributing heat to the inside of the case
-You can tuck extra cabling in the top optical drive slot as long as you leave it empty.. but this could be a con as well as you wouldn't be able to use the top slot in this case, so in my opinion this cancels this one out.
Is there anything else? I can't think of any personally.
So if it weren't true that bottom mounted PSU contribute to heat, then there would not be any reason to use a top mounted PSU at all, right? OK then yes we all know that heat rises, and the heat from the PSU would rise up and heat up the motherboard and other components and then get trapped in the top of the case... in theory anyway, but you forget about airflow. Yes heat rises but it is also easily carried away and dissipated by air, meaning that with some horizontal airflow it will simply be blown out the back of the case. Also, by eliminating the PSU from the top of the case, you can install a large 160mm fan in its place (or in some cases two or three of them). This prevents any heat from building up in the top of the case at all. Also, since these larger fans have larger blades they can spin slower and still provide the same airflow for a lot less noise.
As a result, I am a fan of the bottom mounted PSU. As long as the case is properly designed for it, heat is not a factor and in my opinion a bottom mounted PSU will last long because it is sucking in cool air from under the case, instead of hot air inside of the case. Also, my computer is much quieter and the cables look much better.
If you truly are worried about heat, many cases will allow you to install the PSU upside-down if you want, and the intake will pull air in from the bottom of the case and push it out of the back.
