Originally Posted by DaveLT
Nah. In natural convection that is correct but we're dealing with forced cooling. And also contrary to what many people think dust actually settles on the ground very quickly, airborne dust is usually not good for you to breathe so there won't be much issues with putting a fan without a filter on top apart from finned VRM heatsinks (like the ones on my board) which does collect dust but it isn't much
For water cooling, yeah.
Cheap carbon filter? Black duct tape?
Not sure what you're saying nah to, but any form of heat transfer heat will always move from high to low temp. If you placed the back and top as exhaust, then placed two thermocouples after the fans measuring the exhaust air, you would find a difference of a few degrees on the top fans hence the top as exhaust is much more effective.
It likely won't make much difference anyway, especially if you are air cooling. But with the dust issue I would set the top to exhaust for sure.