PWM fans, +/- pressure - confused

[ Sorry, posted in newbuild forum section at first, so apologies if you've seen it before]


I have recently built a new PC, spec as follows:

MSI Z68GD55, i5 2500K (clocked to 4.5), 8Gb Vengeance RAM, Veliciraptor, Antec Kuhler 620, Cougar GX650 PSU. No discrete graphics yet, using Intel HD3000 for now.

I am trying to sort out my cooling and airflow/cabling etc, what has started me is the very noisy fan that is supplied with the Kuhler unit. I want to replace it, but don't want to do so without understanding the options fully.

My case has 3 intake fan slots - Front 1 Empty, Front 2 Bitfenix Spectre (60cfm, 18db), Bottom Akasa Silent (40cfm, 18db) - all 120mm, all with dust filters. Total input 100cfm.

It has 3 exhaust fan slots, Top 1 & 2 Bitfenix Spectre 140mm (60cfm, 18db), Rear Kuhler 620 supplied fan (80cfm, ?db, LOUD!). Total output 200cfm.

The PSU is bottom mounted, with bottom filtered intake and rear exhaust.

Now, my first confusion is the fan that is supplied with the Kuhler. It is allegedly 1450 to 2000 RPM. I'm not sure how it can manage this as it's a 3 pin non PWM fan, that plugs into the pump unit, which also has a 3 pin connection to the motherboard. So presumable it's stepped at 5 7 12v within the pump etc to get the different speeds. Anyway, I've never seen it budge from 1400rpm, and it's noisy at that (or is the 1400 referring to the pump speed!?).

Anyway, I want to swap it out for a quieter model with similar cooling power, and I am under what may be just received opinion that you are better with a PWM fan for CPU cooling, so it can increase the airflow as you start to work the CPU. However I am yet to find a PWM fan that performs like a good quality 3 pin fan. For example:

Akasa Apache PWM - 600 - 1300 rpm, 57.53 CFM (Presume Max), 7 - 16 dB
Deepcool UF-120 PWM - 500 - 1500 rpm, 66.3 CFM (Presume Max), 17.6 - 27.8 dB
Enermax UCTB12 3pin - 900 rpm, 71.54 CFM, 11 dB

Taking the above examples, neither PWM fan has as much airflow capacity, neither are quieter (at Max), and neither are slower (more vibration). The Enermax fan, as well as being cheaper, will push out more air at lower speeds and less noise than the PWM fans, at the expense of running flat out all the time. Is there any penalty in doing this, other than it seems inelegant compared to a PWM system? Additionaly, surely you have no way of knowing your case pressure traits, if your exhaust cfm total is fluctuating with PWM fans?

Assuming it's a good choice to swap out for the Enermax, I will be running at approx 90cfm more exahust than intake making the case very negative pressure. It's only been running a week and I already have dust build up through the front panel mesh grills in areas other than the fans and filters and would like to prevent this.

At this point in order to introduce positive pressure, I need either another front fan and/or replace the existing once to add at least 100cfm for +10cfm positive, or alternatively I could turn my Kuhler rear fan into intake. There seems to be 2 takes on this, exhausting through the radiator means less efficient CPU cooling as you are cooling it with warm air from the case, but intaking through it will keep your CPU cooler at the expense of passing warm air into the case, but it would at least be directly under both 140mm top exahusts into a largely empty areas of case as there is no massive CPU heatsink. Doing so would put me to 120cfm exhaust, and 170cfm input, so very positive meanign any heat build up will be actively trying to get out.

Anyway, that turned out longer than I thought...Who knew it was so bloody complicated! if anyone has any thoughts on what the deal is with the supplied Kuhler fan, why PWM seems to be pointless, and whether intaking through a radiator and running a minimal or huge cfm positive pressure differential is preferable, I'd gladly read your replies :)

I also need to take into account future changes, like when I put in the 560GTX I intend to use...

If you have read this far, well done, and thanks!
4 answers Last reply
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  1. May I suggest that you use my configuration and do a push/pull fan on the Anetec 620 radiator. I use Cougar fans:

    If you have a fan controller and want 3 pin fans these are the ones I recommend:

    As for positive vs. negative airflow, I recommend that you swap out the 2 intake 120 mm fans for the 3 pin Cougars and add a 3rd Cougar.

    BTW the real rating for the Cougar 3pin fans is 60 cfm (the 102.6 is cubic meters per hour used in europe).
  2. One caution: the Cougars are often sold out at Newegg.
  3. Cheers - I'm in the UK, so have to source from elsewhere, and Cougars are quite hard to get hold of, but I think you are right in that I will likely replace all fronts with the Enermax giving 210cfm intake, and replace the Antec fan with a 4th one to give 200cfm exhaust.

    In the meantime I will try to just turn the Antec fan to intake mode, moving me to 180cfm intake, and 120 exhaust. I''m getting 27 degrees idle as it stands, up to 60 degrees prime 95 which I think is pretty good for a 4.5 overclock (3.6 v peak), and that should go down in that configuration - not worried about the heat going into the case right now, there's nothing in there and a healthy airflow through top vents.
  4. If the Antec is attached to the radiator don't reverse it. It is supposed to be an exhaust fan and will blow the heat into the case over the other components.

    Alternatives to the Cougar fans are Xigmatek fans which are quiet and powerful (but not as good as Cougar). Like this one:

    The best cooling is intake from front bottom and exhaust out top rear. If you have issues with GPU heat then side door fans on the GPU as well.
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