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Determining the correct CFM for an exhaust fan

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March 30, 2009 10:20:28 PM

I have the CM 690 case, it has 3 stock 120mm fan with a 44.73 CFM. I has one on the front, side, and the exhaust on the back. I've covered up all the other port to prevent any leaking. I want to replace all the fan with fans that have higher CFM, 100 CFM for the front and side fans. I have no clue as to what CFM do I need for the exhaust fan. Could someone help me out?
a c 80 ) Power supply
March 31, 2009 2:22:55 AM

We are doing this why?

I wouldn't bother trying to cover up the holes, etc. I bet there are gaps where the 5.25" drive bays are. The question (if you even want to ask it) is do you want a case with positive or negative ventilation? If you have positive ventilation, you'll have more air coming in then out, negative is the opposite ofcourse. There are debates and theories about which is better, I'm not versed enough in either side to write about it.

Here is what I'd do. Remove any covering that you've put in place. Those holes are there for a reason, don't block them. (unless you ARE smarter then the Cooler Master engineers who built the case.) Just remove the stock fans and replace them with something faster/better. Remember that faster fans will output more noise, and those that move lots of CFM tend to output the most. Even if the fan itself is quiet, the moving air can/will be noisy. I'm not even sure if there are any 100+CFM fans, I tend to see 80 max. Even the almighty tornado maxed out at 89.something. Look for something in the 50-80CFM range, and try to keep the dba to around 30.
March 31, 2009 6:26:32 AM

I have the same case.

2x Stock 120mm fans installed in the top air vent as exhaust.
1x Silverstone 120mm, 110 CFM fan as a rear exhaust fan.
1x Stock 120mm fan as intake in the front of the case.

I have been running this for a couple months with no heat issues.

I also have 2x Vantec Tornado's as intake on the side but their disconnected
I will switch them on when I can replace my fan controller and when I need them.
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a c 144 ) Power supply
March 31, 2009 3:29:30 PM

47... is right. We are doing this why?

Build your system with the case fans' stock configuration. Test. Overclock. Test some more. If you have cooling problems, then think about upgrading the fans.

I originally had a Q6600 with a TRUE OC'd to 3.6 GHz. in an Antec 900 case. Running Prime95 with all the case fans set to LOW, my CPU temps ran 61 C. to 65 C. With the fans set to MED, my CPU temps were ... 61 C. to 65 C. With the fans set to HIGH, my CPU temps were, yes, indeed, 61 C. to 65 C. And the computer sounded like a freakin' vacuum cleaner sitting on the table.

Depending how efficiently your case moves air through it, there will be a point where more cfm does not equal more cooling.
March 31, 2009 4:22:19 PM

4745454b said:
We are doing this why?

I wouldn't bother trying to cover up the holes, etc. I bet there are gaps where the 5.25" drive bays are. The question (if you even want to ask it) is do you want a case with positive or negative ventilation? If you have positive ventilation, you'll have more air coming in then out, negative is the opposite ofcourse. There are debates and theories about which is better, I'm not versed enough in either side to write about it.

Here is what I'd do. Remove any covering that you've put in place. Those holes are there for a reason, don't block them. (unless you ARE smarter then the Cooler Master engineers who built the case.) Just remove the stock fans and replace them with something faster/better. Remember that faster fans will output more noise, and those that move lots of CFM tend to output the most. Even if the fan itself is quiet, the moving air can/will be noisy. I'm not even sure if there are any 100+CFM fans, I tend to see 80 max. Even the almighty tornado maxed out at 89.something. Look for something in the 50-80CFM range, and try to keep the dba to around 30.

jsc said:
47... is right. We are doing this why?

Build your system with the case fans' stock configuration. Test. Overclock. Test some more. If you have cooling problems, then think about upgrading the fans.

I originally had a Q6600 with a TRUE OC'd to 3.6 GHz. in an Antec 900 case. Running Prime95 with all the case fans set to LOW, my CPU temps ran 61 C. to 65 C. With the fans set to MED, my CPU temps were ... 61 C. to 65 C. With the fans set to HIGH, my CPU temps were, yes, indeed, 61 C. to 65 C. And the computer sounded like a freakin' vacuum cleaner sitting on the table.

Depending how efficiently your case moves air through it, there will be a point where more cfm does not equal more cooling.


I want a positive ventilation so that every part of my case is reached. I've also covered up the left over 5.25" drive bays and I covered them up to direct the air to the right place on my case to increase air pressure. I don't want cool air exiting too soon through open ports, and not being able to reach other parts, nor do I want an open port causing a low pressure situation where in places where I would need the full pull of a fan concentrated on the air being pushed in for example. I just want to figure out how to balance my CFM. Do I add up all my intake CFM and then that's my exhaust CFM?

I'm upgrading my intake fans to 100+ CFM to directly cool my HHD and Graphics card as they tend to heat up, especially my Graphics card. The front fan is directly in front of my HHD which leaves not enough cool air pressure for the rest of my case after it passes through the HHD cage, even with the help of a temp 120mm fan blowing from the bottom up. The side fan is for directly to my Graphics card and it's not cooling it enough.

As far as fans with 100+ CFM, they have ones that go up to 256 CFM from Delta fans from FrozenCPU.com.

ravenware said:
I have the same case.

2x Stock 120mm fans installed in the top air vent as exhaust.
1x Silverstone 120mm, 110 CFM fan as a rear exhaust fan.
1x Stock 120mm fan as intake in the front of the case.

I have been running this for a couple months with no heat issues.

I also have 2x Vantec Tornado's as intake on the side but their disconnected
I will switch them on when I can replace my fan controller and when I need them.


I was thinking of doing that and hesitated, I ended up with my current setup from reading around. By the way what CFM are your Ventec fans?
a b ) Power supply
March 31, 2009 4:51:52 PM

if you want positive, the combined cfm of the intakes should be higher than the cfm of the exhaust.

but the way I've understand it, the point with positive airflow is to let the air leak out at every slot and opening in the case, so there isn't any 'dead' spots with poor airflow. And if you have dustfilters on the intakes, there would be minimal amount of dust build up within the case since the air (and dust) isn't being sucked in through those misc slots and holes as it would be with negative pressure...

browsed a little at frozencpu and found this
http://www.frozencpu.com/products/7866/fan-481/Delta_120_x_38mm_Extreme_High-Speed_Fan_-_22001_CFM_TFB1212GHE.html?tl=c15s780b113#blank
yikes, fan thats rated at 35W, I dont wanna know how loud that thing is. 0_o lol
March 31, 2009 6:33:45 PM

Kari said:
if you want positive, the combined cfm of the intakes should be higher than the cfm of the exhaust.

but the way I've understand it, the point with positive airflow is to let the air leak out at every slot and opening in the case, so there isn't any 'dead' spots with poor airflow. And if you have dustfilters on the intakes, there would be minimal amount of dust build up within the case since the air (and dust) isn't being sucked in through those misc slots and holes as it would be with negative pressure...

browsed a little at frozencpu and found this
http://www.frozencpu.com/products/7866/fan-481/Delta_120_x_38mm_Extreme_High-Speed_Fan_-_22001_CFM_TFB1212GHE.html?tl=c15s780b113#blank
yikes, fan thats rated at 35W, I dont wanna know how loud that thing is. 0_o lol


So I just add up all my CFM for intake to get an estimate for the exhaust?

Deltas were the fans I was going to go with for my exhaust.
March 31, 2009 6:57:48 PM

I have a Delta fan. It is LOUD. In a way that no other case fan can match.

Go with lower (~80CFM at most) fans if you care at all about noise. I'm not talking about dead silent either - I mean if you don't want your computer to sound like a vacuum cleaner. Don't bother trying to seal the case if you are going for positive pressure, and as said before, just make sure that your total intake CFM is significantly above total exhaust CFM.
a b ) Power supply
March 31, 2009 7:07:50 PM

I have installed a in/out thermometer which sits on by desk. The "Out" thermistor is mounted about 2 inches infront of the CPU cooler fan. With this you can try to optimize the fans for the coolest temp.

Just a note on positive pressure: As the pressure on the inside of the case increases, the air intake of your "100 CMF" fan will be reduced. If you did a real good jof of sealing the case your intake could be reduced by 25% or more. Myself, I did block off the top ventilation of my CM 690 so that more lower/cooler air is sucked in. I also found that close to a balanced airflow (Just slightly +) resulted in the coolest temp. After being powered on for 7 hrs my inside case temp is 70.7 F and room temp is 70F ( top of desk is 72F).
a c 248 ) Power supply
March 31, 2009 7:39:47 PM

+1 what the RetiredChief said about positive pressure. It doesn't take very much to create positive pressure.

I hope you realise you will be creating a pc that is going to sound louder than a jumbo jet. As you can see from the comments made by other veteran posters, you are getting carried away and going way beyond the point of diminishing returns. There are better ways to improve ventilation, air flow, and cooling.

In your posts you mentioned high temperatures.
What is the ambient room temperature?
What cpu do you have?
What is the cpu temp at idle?
What is the cpu temp at full load?
What are the cpu core temps at idle?
What are the cpu core temps at full load?
What cpu heatsink do you have installed?
What thermal compound did you use?
What software are you using to measure all those temps?
What video card do you have?
What is the gpu temp at idle?
What is the gpu temp at load?
What software are you using to measure gpu temps?
March 31, 2009 9:18:06 PM

JohnnyLucky said:
+1 what the RetiredChief said about positive pressure. It doesn't take very much to create positive pressure.

I hope you realise you will be creating a pc that is going to sound louder than a jumbo jet. As you can see from the comments made by other veteran posters, you are getting carried away and going way beyond the point of diminishing returns. There are better ways to improve ventilation, air flow, and cooling.

In your posts you mentioned high temperatures.
What is the ambient room temperature?
What cpu do you have?
What is the cpu temp at idle?
What is the cpu temp at full load?
What are the cpu core temps at idle?
What are the cpu core temps at full load?
What cpu heatsink do you have installed?
What thermal compound did you use?
What software are you using to measure all those temps?
What video card do you have?
What is the gpu temp at idle?
What is the gpu temp at load?
What software are you using to measure gpu temps?


Yeah you're right. But I do want to upgrade my fans from my stock CFM. When I take out my HHD it's hot when touch and the video card casing is even hotter.


Intel E6750
Around 31 C
Never tested it


Stock but upgrading it to the Zalman CNPS9700 LED
Stock but with the upgrade, Arctic Silver Ceramique
Nothing yet, just touching it after running on it for sometime(any suggestion on software)
EVGA 8800GT
Around 52 C
Never tested it


Nothing yet, just touching it after running on it for sometime(any suggestion on software)
March 31, 2009 10:16:18 PM

I recommend that you use Speedfan to tell your HDD temps, use Real Temp or NVMonitor to tell your GPU temps, and Core Temp, Real Temp, or Speedfan to tell your CPU temps.

Your CPU is ok in the range of toward ~70C. More than that at load would indicate a definite need for better cooling.

Your GPU will be hot. Anywhere between 45C (idle) to upwards of 80C (load) is normal for an 8800 series

Your HDD should be between ~40C to 55C during normal running.

Is 31 degrees your ambient temperature or your processor temperature? If you have a 31C ambient, you may want to consider water cooling.
March 31, 2009 10:26:22 PM

Groveling_Wyrm said:
I recommend that you use Speedfan to tell your HDD temps, use Real Temp or NVMonitor to tell your GPU temps, and Core Temp, Real Temp, or Speedfan to tell your CPU temps.

Your CPU is ok in the range of toward ~70C. More than that at load would indicate a definite need for better cooling.

Your GPU will be hot. Anywhere between 45C (idle) to upwards of 80C (load) is normal for an 8800 series

Your HDD should be between ~40C to 55C during normal running.

Is 31 degrees your ambient temperature or your processor temperature? If you have a 31C ambient, you may want to consider water cooling.


31C is my processor temperature. 31C for ambient high to have to go to water cooling? Is there one software with all that measurement in one?
March 31, 2009 11:44:23 PM

I have Cm-690 and i have 3 stock fans placed at the top 2 slots and the rear exhaust , all exhausting air , meanwhile i have duct taped the the vents on the side panel on both sides .With this setup i have all fans on min , basically silent and my CPU never goes over 45C and my video hovers at 45C aswell under full load.At max speed everything runs at 30 or lower at full load.No need for a jet engine for a case jus test things out , with my current setup i have reached the coolest system as of yet , nevermind it's basically silent.


Cooling setup:
Coreduo2 e6750 OC'd to 3.2 with Thermaltake tower heatsink
8800 gt 512 oc'd 710/935 with Arctic Cooling accelro S2 heatsink with a low speed 120 fan zip tied to it
Ambient 20C
Also some decent cable management
April 1, 2009 9:58:37 PM

I might have to put more exhaust fans to be able to balance out my intake. I can put one exhaust but as you guys say, it will be loud.

Is there a software that will read the temperature of your CPU, HHD, and Video card and tell you the temperature and then has a set of boundaries for speed percentage for the fan; if the temperature is above a temperature it will increase the speed and then if below that temperature it should just stay at a stationary percentage speed? Or maybe a fan controler with temerature gauge for those components and ambient temperature?
a c 139 ) Power supply
April 1, 2009 10:59:25 PM

SiDE said:
I might have to put more exhaust fans to be able to balance out my intake.
Doing anything without measuring the stock results could end up leaving you with a worse performing case.
Those measurements will tell you what, and how much, you need to change things around.
And because you know the stock temps - you'll know if the changes are an improvement.
Having higher input than exhaust isn't a bad thing if it's helping to cool your system better.

a c 248 ) Power supply
April 1, 2009 11:45:00 PM

+1 what WR2 said
!