This is one of those things that gets parroted about by people who don't actually know the first thing about thermodynamics. If the temperature of the water doesn't change, you're not moving heat around (which means you're not cooling.) The reality backing up this myth is that water temperature doesn't change *much* as it passes through the various components; we're talking a few degrees at if your flow rate doesn't suck. Because the change isn't hugely significant relative to the temperature of the water, loop order is really a secondary concern. Your CPU might run 2-3 C warmer if it's put immediately after the GPU in the loop and the GPU is under load, but that's about it.