Do I need two fans on the radiator of my CPU?

Question

I ordered a water cooling all-in-one solution for my CPU and I was wondering if I really need both fans that come shipped with it, or if one fan is enough.

What I am talking about is one of those coolers, where there is a fan on both sides of the radiator:

It seems to me that the second fan doesn't have a lot of work since the first fan already got the air moving and I don't see it slowing down that much inside the radiator, or am I wrong here?

Can I just leave off one fan and have a similar performance and if yes, which one should I keep? If no, why are two fans necessary or to be recommended? I am planning to mount it at the rear side of the case next to the IO-panel.

Solution

This is called a push-pull fan configuration.

You can remove one of the fans if, for example, the radiator/fan assembly would otherwise be too thick to fit in the case, but this will reduce cooling performance. The double fans help concentrate airflow through the radiator fins; a significant amount of static pressure is needed to push through the narrow fins, so having two fans increases effective airflow. The alternative would be to run a single, faster fan optimized for static pressure, but this would generate more noise.

If the fans supplied with the liquid cooler aren't satisfactory, you can, in most cases, replace them with aftermarket fans of your choice. If you're planning on doing this, be sure to choose fans optimized for static pressure—a good pick would be the Corsair SP series.

Comments

[David Schwartz]

I think you're basically right. The difference between a single fan and two identical fans in a push-pull configuration is usually just a few degrees. Unless you're trying to get every possible drop of overclocking out of your CPU, you probably only need one.

[Ramhound]

It entirely depends if your willing to accept the performance difference. To know how much that difference is, benchmark both configurations, then make a decision.

[comfreak]

@DavidSchwartz So which fan would you leave out? The one between the case and the radiator or the other one on the inside?

[comfreak]

@Ramhound Do you have any rough guess or some numbers from your experience how much that typically is? I am not (yet) planning to overclock, my main goal is to reduce temperatures and make the whole computer more quiet.

[Ramhound]

Every cooler, case, and CPU will be different. Why don't you just benchmark it yourself since its your hardware?

[David Schwartz]

@comfreak The fan that pushes air is much more efficient than the fan that pulls air. So you want the fan that pushes air into the radiator.

[davidgo]

If quietness is your goal, would you not be better off with running both fans at lower RPM ?

[Moab]

You need one of these...pugetsystems.com/submerged.php

[davidgo]

why would the fan which pulls air be a better choice then the one which pushes air ? Based on the observation that cars have the fan pushing air through the radiator I would have thought that pushing air would be the more efficient solution. [ from my limited knowledge working with pumps pushing fluid this also avoids cavitation, however I suspect cavitation is not very relevant in air ]

[Goodbye]

Simple. The CPU temp will be equals to the division of Temp with Hot Air pulled out. If youbput air: you will be adding ambient temperature to it. Wich in 90% situations will increase the temp. While if you extract air, you will take temp, not otherwise.

[Goodbye]

Current CPU Core Temp : Extracted Air Temp = The Cooling Temp. Or Current CPU Core Temp : Heat Sink Temp = Real Temp

[davidgo]

youtube.com/watch?v=UyC3lZ5WFMk is a compelling argument that push vs pull makes little difference to cooling / performance - but mount the fans in such a way that they are easiest to clean. Other resources like martinsliquidlab.petrastech.com/… show a slight benefit in push over pull. I confess that I don't understand your logic about ambient air temperature. In both cases you are moving air through an open system.

[Goodbye]

Also a car will be in motion. The computer wont. So if you put air in the cpu you will had ambient temp. (Therefore a hot ambient will give you heat to CPU - a cold ambient... cold air). So for me in a almost tropical climate it is best to take air than put. I can take 30 degrees celcius from my CPU That way. If o

[Goodbye]

If I put air in: The cooling will always be higher. Than if I extrwct heat.

[David Schwartz]

Pretty much all that matters is how much air flows over the fins. That will be determined by the pressure differential the fan is able to create. You can create a bigger pressure differential by blowing than by sucking because when you (or a fan) sucks, you are limited by the amount of ambient air pressure. There is no such limit when blowing. So you want the high pressure side of the fan towards the constriction. Make your fans blow, not suck. (Except when you need to equalize pressure with multiple fans or other special situations.)