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Airflow Conundrum!

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January 22, 2010 1:07:21 AM

I've attached an image that won't do it justice, but I've got the new Corsair Obsidian 800D Case, and added some accessory fans. That is an understatement.

I've popped some YS Tech 120x38mm Fan - 126CFM, a Noctua NF-P12 120mm x 25mm Fan - 54.3 CFM on my Thermalright Ultra-120 eXtreme-1366 RT Rev. C CPU Heatsink, as well as that same Noctua down below on the non-hotswap bay.

Bottom line, my rear fan doesn't spin. Not enough air it would seem, as the 38mm's are removing air at a rate too fast to comprehend.

Can too much fanpower be a bad thing? Should I replace the 38s with some 25s (like these)?



Oh yeah, the YS Tech fans are INSANELY LOUD! I can here them in adjacent rooms and one floor above (especially at night).

More about : airflow conundrum

a b B Homebuilt system
January 22, 2010 1:22:45 AM

I would replace your back fan. It should spin reguardless of air flow since it has a "motor" spinning it. It isn't driven by airflow.

Too much fan power can be a bad thing if it effects the airflow balance within the case. Meaning the hot air doesn't get blown out of the case but recirculates within the case.
a b B Homebuilt system
January 22, 2010 1:44:37 AM

is the fan in the non hotswap bay acting as an exhaust? If so flip it around, it works best to bring cool air in the bottom front and exhaust it out the top back, that way you arent fighting the tendency of hot air to rise. If the back fan isnt spinning, its motor is dead, if the air was getting exhausted too fast it might spin slowly but it would still spin.
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a c 83 B Homebuilt system
January 22, 2010 2:00:45 AM

What is your problem? Noise / high temperatures or other?

1) verify that all of your fans are sending air in the proper direction. Dangle a piece of tissue in front of it to test.

2) In general you want the airflow front intake to back exhaust, and low to high.

3) If a fan does not spin, it is because it does not get sufficient voltage to start. Have you overloaded the motherboard fan headers? There are limits to the amperage they can handle.

4) Is your true exhausting hot air out the top of the case, or to the rear?

January 22, 2010 1:26:27 PM

Here's another graphic for you:



The original image is from a case review from a competitor, but I doctored it to my specific situation.

All the fans are pointed in the right direction, and the RED STAR fan spins very slowly, 30-45RPM ballpark (I can see the blade slowly spinning like something you might see at some kind of industrial style club where the fan blades are projected on the floor). When I remove the side of the case with the window, the rear fan spins as normal.

I do have a Fan-Adapter in the system as well, so it isn't all coming off the mother board. Both stars are CORSAIR PROVIDED fans, I bought the wrong replacements for them so left the OEM fans in those positions.

I should add that the original Cooling Zone 2 fan which exhausts out the side (odd to me, since cables are routed back there) is also a corsair 140mm fan, and just below that there was a spot for an accessory fan which I did add.

In terms of your other questions:

1) They are all pointing in the intended direction, allowing heat to flow in a natural direction (up and out) and intake from the coldest source (bottom/front).
2) The case is not hot, and the cards do NOT at all get hot, at load my 5870 gets to maybe 60*C? Right now it is sitting at 42*C as I browse web. I just want to get that back fan spinning or doing something!
a c 83 B Homebuilt system
January 22, 2010 4:04:24 PM

Interesting...

First of all, I do not see a problem that needs to be fixed. Your temperatures are nominal.

Why the rear fan does not spin is curious, particularly why it spins with the side cover off.

If the rear fan is plugged into a molex connector, it should spin at it's rated speed, regardless.

My guess is that the rear fan is controlled by some sort of thermal sensor and will not turn on unless the temperature it senses gets high enough.
This could be a sensor in the fan itself, or something in the fan adapter(what is it?) .
It rotates slowly soley because of the strong airflow exiting the case.

When you remove the side panel, you disturb the strong bottom to top airflow and increases the temp that the rear fan is sensing.

Try connecting a different rear fan directly attached to a molex connector.
January 22, 2010 4:08:42 PM

I will order some replacement 140's to go on the case, the REAR FAN IN QUESTION /is/ plugged into the motherboard. I will replace it and update this thread. It seems to not spin because of the air pressure? If I leave the SIDES ON, and place my hand over the perforation behind the REAR FAN, it also can begin to spin at speed. Thanks to everyone for the replies so far!
a c 83 B Homebuilt system
January 22, 2010 4:18:48 PM

In the motherboard bios, you might find some options on controlling motherboard fan headers. I don't like my fans to adjust their speed at all. It seems more intrusive to me to have them constantly adjusting than a steady drone. I pick fans or set speeds to give adequate cooling at a reasonable noise level. If things get hotter under load, so be it, just so the temps do not exceed maximum acceptable levels.

You might want to install speedfan to see what rate your motherboard controlled fans are running at.
http://www.almico.com/speedfan.php

I use it mainly to display a temp icon in my taskbar.

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a b B Homebuilt system
January 22, 2010 4:38:03 PM
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geofelt said:
My guess is that the rear fan is controlled by some sort of thermal sensor and will not turn on unless the temperature it senses gets high enough.


^^ This. This is my guess if the rear fan is plugged into one of the motherboard headers. The other fans might be doing such a good job that the motherboard doesn't think it needs to use that fan. I had one machine where the case cooling was just insanely good and the motherboard turned the CPU fan down so low that I could see it making each individual spin, which was making me think it was broken.

If you have a fan controller that's independent of the motherboard, plug the rear fan into that and see if it works as it should. Alternately, there's usually a setting in the BIOS where you can just let the fan run instead of having it controlled automatically. Or if not the BIOS, there may have been monitoring software that came with your motherboard that lets you make the same kind of adjustments.n
December 4, 2011 12:32:48 AM

Best answer selected by wshinds.
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