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Fan Cooling Options (Pressure, PWM) - confused

Last response: in Systems
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December 14, 2011 2:49:37 PM

[ Sorry, posted in wrong section, so cross posted in Overclockers/Cooling section, feel free to reply there is it's better placed]

Hi,

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!

...so 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!
December 14, 2011 4:00:39 PM

You have properly analyzed your situation; now what to do.
What case do you have?

1) The main problem seems to be the high fan noise on the kuhler. I think that comes from the temperature sensors in the unit thinking that the full 1400 rpm is needed. Do you get 1400rpm at idle? If so, I think you should not, and perhaps a support request to Antec is in order. You might try their recommended install of using the rear as an intake to the kuhler. That way the cpu should be cooler and require less rpm. I do not think I would try to mess with replacing the fan, since it seems to have been engineered for the unit.

2) For the reasons you stated, I do not like the liquid all in one coolers unless you have a space issue. For $30 or so, you could return/sell the kuhler and replace it with a cm hyper212 or a Xigmatek gaia. The fan would then be under motherboard control.

3) You can definitely help the positive pressure situation by adding another front intake fan. The exhaust seems sufficient to take care of itself.

4) Do you really need a 4.5 OC? That generates lots of heat. Would not a more modest 4.0 do just as well?

5) If you have not yet bought a graphics card, look for one with a reference cooler that exhausts the hot air out the back of the case. Aftermarket coolers do a great job on an open testbed, but then dump the heat back into the case where your case cooling has to deal with it.
Here is an example of what I mean:
I like this: http: //www.newegg.com/Product/Product.aspx?Item=N82E1681413066...
better than this: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

6) Your parts are not high heat generating, and are quite tolerant of higher temperatures. Do not worry about elevated temperatures too much.
If temperatures get dangerously high, the cpu or gpu will downclock to preserve themselves.
December 14, 2011 6:44:11 PM

Cheers very much, helpful answer

>What case do you have?
Bitfenix Shinobi (no window & side vent))

> The main problem seems to be the high fan noise on the kuhler. I think that comes from the temperature sensors in the unit thinking that the full 1400 rpm is needed. Do you get 1400rpm at idle?
the 1400 is the lower figure, and I always get it. I set the fan to half speed, and it dropped by like 10rpm max. I think it's referring to the pump. I am abut to disconnect the fan and leave the pump running and see what the bios says.

> For the reasons you stated, I do not like the liquid all in one coolers unless you have a space issue. For $30 or so, you could return/sell the kuhler and replace it with a cm hyper212 or a Xigmatek gaia. The fan would then be under motherboard control.
Going to put the fan direct onto the CPU fan connection, rather than run it through the pump, and see what speeds it says.

> Do you really need a 4.5 OC? That generates lots of heat. Would not a more modest 4.0 do just as well?
I'm not sure, I did it because it took it so well. 26 degrees idle, and 60 on prime 95 with 3.6 v peak.

> If you have not yet bought a graphics card, look for one with a reference cooler that exhausts the hot air out the back of the case. Aftermarket coolers do a great job on an open testbed, but then dump the heat back into the case where your case cooling has to deal with it.
Sure, my airflow is very clean at the moment, all passing front to back and bottom to top without any "splash" from other directions.

!