Does the order of the loop matter for water cooling?

This is a question you will often see online, and while the short answer is no, the long answer is yes.

To give a proper answer to this question, we need to first conduct a few basic tests. Let's do this with a system with pumps running at a very slow speed, so the flow through the system is not going very fast (in this case, about 60 liters per hour). When the coolant is flowing slow through the system, it will pick up more heat from the individual components, and the next component in line will thus get warmer coolant.

In our example system, the coolant heats up by about 5 degrees Celsius after passing over our RTX 4090 running furmark when the coolant is only running at 60 l/h.

Now, let's try and set the flow higher – what will happen? Turning up the flow rate means that the water now has less time to be heated up by the graphics cards as it will flow faster through the waterblock, having less time to extract heat from the hot cold plate. Our flow is now 170 l/h and the difference in temperature from the water going into the graphic card block, to the water going out of it is now down to 2 degrees. Imagine running the cold tap from your sink over a hot frying pan - if you run the water very, very slowly, it will be warmed up instantly, but if you on the other hand run it at full speed you will, on top of destroying your frying pan, notice that the water coming off the pan barely heats up.

What we can learn from this, is that we can have a few loop examples:

• Loop 1 that goes from the radiator -> graphics card -> processor, and have pumps running at very low speeds
• Loop 2 has the same setup, but with pumps at high speeds
• Loop 3 goes from radiator -> processor -> graphics card

In loop 1, the graphics card will heat up the coolant significantly before it reaches the CPU, and pretty much every extra degree the coolant is equals an extra degree for the CPU temperature. In this example, the CPU temperature would be 5 degrees higher than if you had used loop 3, or 3 degrees higher than if you had your pumps running at high.

Why does this matter? CPUs are typically the components that will becomes the warmest in a system as they have such a small surface area to dissipate of the heat. It is not uncommon for modern day high-end CPUs to reach 85+ degrees, and when you get into those ranges, lowering your temps an extra 3-5 degrees just by having your CPU come first in the loop order is a no-brainer. Since a watercooled graphics card will typically be around 40-60 degrees, it is much less critical that this component be cooled with the absolute coldest coolant possible.

So, is it the end of the world if the CPU does not come directly after the radiators in a loop? Not really, there are plenty of situations where the order of the loop will simply look much better, or be easier if you have the graphics card first, and that is also OK, but if you are chasing every single degree you can get, then putting the CPU before the GPU could help a little bit.