While the people here pointing out the much higher heat output of GPUs are of course right, they are also ignoring other important factors in cooling, such as thermal density and interface efficiency. GPUs put out tons of heat, but are cooled direct-die and have their thermal load spread evenly across the vast majority of the die. CPUs produce less heat, but have IHSes that impinge on thermal transfer in the name of safety and ease of installation, and have dramatically higher thermal density than GPUs thanks to the cores being a relatively small part of the die. This means CPUs are much more difficult to keep cool overall - as demonstrated by the numbers posted above by people talking about prioritizing their GPUs in their loops. A GPU on water will run cool almost no matter what - remember how AMD's previous water cooled reference designs have been? Cool, quiet (unless you got a whiny Fury X like me), and all with a 120mm rad. A 240 will of course run cooler or allow for slower fans, but a 120 can still handle a 300W GPU with a good fan. There is no way whatsoever you'd be able to cool a 300W CPU with a 120mm rad, no matter the fan - it would be thermal throttling long before hitting those power levels. CPUs simply aren't able to dissipate their heat efficiently enough, and thus require as much attention in a custom loop as the GPU despite the much lower thermal output in watts. After all, what matters isn't the heat output, but the temperature of the die. Everything else is there to control that one thing. And at any given wattage, a GPU will always be easier to cool than a CPU.
