I'm pretty sure NZXT says in their manual to mount it with the tubes on top. My understanding is that with this type of radiator, the liquid must flow from one hose, all the way to the other end of the radiator, then back toward the other hose and then back toward the pump. If the radiator is mounted with tubes at the bottom and air bubbles form, and they always do, closed loop or not, then eventually you will have an air pocket at the top of the radiator. This would not allow the water to travel from one side of the radiator to the other and you end up with stagnated water in a loop that should be moving it in a loop. This is just my understanding. I might be wrong.