We have to get away from the custom liquid cooling theme a bit with this aspect in the AIOs. And it must be done for several reasons of pure logic: thermal performance. This also includes a series of elements that affect the performance of each unit, so we are going to deal with them to answer the initial question.
Why isn’t the performance of an AIO liquid cooling scalable to bigger radiators?
In custom liquid cooling, a larger radiator in terms of length or width will always be better than a smaller one in these parameters, something that we already explained in a specific article and that we will ignore the reasons for this to leave that reading for the most curious. In an AIO liquid cooling we have a series of limitations that, on many occasions, have given surprises when testing the units against their competition.
Bigger therefore is not synonymous with better, it cannot be taken as dogma in an AIO and the reasons are varied. In the first place and as a fundamental piece we have the system pump. We speak, as a general rule and except for the “somewhat custom” models, of pumps between 6 and 8 watts with flows that in no case exceed three figures in terms of liters per hour, in fact they stay much closer to the digit than to exceed 3.
What about this? Well, a radiator and its performance is almost proportional to the flow of water that passes through it. You can change the air flow by changing the fan and gain performance, but you cannot change the pump, ergo, a larger surface in terms of width or length reduces performance, since common pumps are not improving at the same level as the rest of the system.
A radiator with a larger surface area implies that the performance of the pump has to be greater to maintain the flow in it, which added to a greater use of fins on the cold plates it gives us an endemic problem in these systems.
Better fans only alleviate the problem
This performance deficiency from current pumps to ever-increasing radiators can be alleviated with better fans, and in fact it is. If you have noticed, in the AIO many times the same fan is not repeated even though the models belong to the same series. In some cases, LED fans and in others not, where the range of RPM, mmH2O and CFM it is totally different, as well as the sound.
This comes from everything mentioned above, since if your pump cannot move enough liquid, your radius is very restrictive or the block has a very high restriction, the performance you have to get from another side and in this case the easy option is include a higher performance fan.
Therefore and taking this into account, is buying an AIO liquid cooling with larger radiators correct, optimal and recommended? Not always, in these systems, as we have seen, you have to look at other factors and sometimes more is not better.