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I have a fairly large aftermarket cooler (Xigmatek Gaia) for my i5-2500k processor. It shipped with one fan (pushing air into the cooler) but has the ability to put a 2nd fan on it (push-pull configuration). I believe the fan is starting to fail, so I'm going to replace it.

My question is, should I go ahead and get two fans for it? I know in cooling the thought is "more is better" but I'm having a hard time believing that two fans blowing an equal volume of air would somehow cool better than just the single fan. My attempts to Google the question were stymied by large amounts of marketing with little info about actual effectiveness, especially in the 1 vs. 2 area. Anyone out there have some know-how about it, especially with regards to the physics of cooling/airflow?

For the record, I am not overclocking now because I have concerns about the fan, but I have in the past and probably will in the future.

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  • why would you believe the two fans would move the same amount of air ? Surely the only way that'd be true is if the second fan did no work at all ?
    – Sirex
    Sep 11, 2012 at 1:07
  • 2
    A single low-torque, low-speed fan (e.g. the super-quiet variety) might benefit from the addition of a "puller" fan on the other side of the heatsink. "More is better" - False, more fans are not better, especially if all you are doing is churning the air within the case. There are three key issues: providing adequate intakes for cool air, providing adequate outlets for hot air, and preventing dead zones where hot air might collect. Whether the inlets and/or outlets use forced convection (i.e. fans) is up to you.
    – sawdust
    Sep 11, 2012 at 1:13
  • Whatever it takes. Sep 11, 2012 at 1:31
  • @Sirex Here's my view: The fan I'm looking at moves ~70cfm. If I put one of these on, it will push 70cfm into and across the cooling fins. If I now put a duplicate fan pulling from the other side, it's going to pull that same 70cfm that the first fan pushed into the fins. It would have more power (i.e., if the air were suddenly thicker and harder to move) but would not pull a greater amount of air or move it any faster. No?
    – techturtle
    Sep 11, 2012 at 3:38
  • the way i see it, that 70cfm of air is now already moving, so the second fan can use it's own power to spin at a higher rpm and accelerate that air faster. Assuming these two fans aren't on a linked rpm. If they are then yeah, you'll gain nothing. sawdust makes a good point though, if the air isn't then directed out of the case, there's little purpose in this.
    – Sirex
    Sep 11, 2012 at 3:49

2 Answers 2

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Since no one was able to provide a concrete answer, I took this to physicsforums.com and asked them. There was some debate there as well, but the end result I got was that two fans definitely can help. The reason is that the second fan increases the pressure along the length of the radiator. One of the regulars there stated this:

The general rule of thumb is that fans in series double the pressure at the same flow and fans in parallel double the flow at the same pressure.

Basically, if the radiator were a straight, frictionless tube with no gaps, then this rule would apply exactly and there would be no benefit to a second fan. However, in the real world, the radiator is far from perfect. There are blockages on the edge of the fins, friction as it flows along the fins, and lots of places for the air to leak out along the way. What the second fan actually accomplishes is not increasing the overall airflow, but making sure that more of the air flows smoothly along the entire thickness of the radiator.

The increased pressure would allow it to keep the speed up by overcoming some of the resistance of friction. That small volume of air that would have hit the first fin and blown out sideways will now be more likely to stay within the radiator, continuing to gather heat along the entire fin. This explains why hardware/overclocking sites and manufacturers report modest, but not phenomenal, gains by adding a second fan in push-pull configuration.

Thanks to everyone here and at Physics Forums who weighed in. It was an enlightening discussion.

(I did go with two fans because I found a great sale on them.)

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  • 2
    I don't believe pressure will be doubled. though it will be slightly increased.
    – Fennekin
    May 13, 2017 at 6:28
  • If your current fan is on the way out but still working, then you can make an easy experiment and take new one you want to replace, put them in series and measure whether it gives you any better results than better of them (fans I mean). I could argue that two fans in series will not be better then one of them. Reason is simple - where is the second going to get the air from? From the first one! If first one is "weak" second is not going to get any better, if the second one is "weak" then it's not going to do any better than the first one either as it will get only this much of air.
    – tansy
    May 12, 2022 at 17:59
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There is very little benefit of the "pull" fan in a "push-pull" setup. It slightly increases the flow thru the radiator (like 10%) but it doesn't translate to any where near 1% better cooling. Instead, you should place that fan at an exhaust slot on the case to suck the hot air out of the case. The only case that the second fan (or series of "pull" fans) would provide a benefit to redirect airflow from the source to pass thru a series of components. An example with an old case from Athlon/Pentium 3 era: From the bottom front of the case to the GPU (itself has fans) and the CPU (thru CPU fan and the exhaust fan at the back of the case following the cpu cooler). So the source airflow will split into two with one going to the GPU and one going thru the CPU radiator and exiting the case via exhaust fan. In short, you only see big benefit from a push-pull fan setup in an semi-open airflow style, not in a tube-like style.

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