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 Post subject: Small fan blocking or helping big fan?
PostPosted: Mon Dec 27, 2010 8:25 am 
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Joined: Tue Feb 06, 2007 1:38 am
Posts: 10
I'm currently trying to silence a SFF HP Compaq (dc7800) to use it as a HTPC. It's really nicely built, with intelligent airflow design, but comes with incredibly loud fans and/or lousy PWM algorithms.

Anyway, it looks like this inside
Image

What I've done so far is, among other things,:

1) Replaced the 92 mm intake fan (to the far left in the beginning of the duct towards the CPU heatsink) with an Arctic Cooler F9 PWM.
2) Removed the 70x10 mm fan (on the opposite side of the CPU) heatsink.
3) Installed a passively cooled PCIe graphics card (in the PCIe slot on the right).

What I'm wondering is: was it a bad move to remove the 70 mm fan?
The logic behind the removal was that it was just blocking the way for the much more powerful 92 mm fan on the other side. Comparing the CFM values (which I don't know for the OEM 70 mm fan) would most certainly reveal that it moves much less air at comparable noise levels than the 92 mm Arctic Cooling fan.

But is the "air blocking theory" valid?

I can't really feel any air moving at all on the right side of the CPU heatsink when only running the 92 mm fan at acceptable noise levels. I gather it would be pretty nice to at least run the 70 mm fan at low speeds (using a fanmate), to blow some air over the passive PCIe graphics card.

So, ro sum it up:

Will a 70 mm, low-RPM fan on the right of the heatsink, make the job harder/easier/no difference for the 92 mm fan on the other end of the air duct?

Thanks for your help! :)


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 Post subject: Re: Small fan blocking or helping big fan?
PostPosted: Mon Dec 27, 2010 9:15 am 
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Joined: Thu Nov 19, 2009 10:20 am
Posts: 482
Location: Ottawa
First off check the BIOS for fan speed settings. I have a similar model and it's not bad with the stock fans if you set the fan speed to min. Default is middle and that's pretty loud. With the fans at min then the 70 shouldn't be loud at all. The BIOS setting also controlled the PSU fan, so you'll quiet that down as well.

Either way just run some stress tests with and without the 70 and check your CPU and GPU temps. If they are too high without the 70 then put it back.


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 Post subject: Re: Small fan blocking or helping big fan?
PostPosted: Mon Dec 27, 2010 12:24 pm 
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Joined: Tue Feb 06, 2007 1:38 am
Posts: 10
Thanks for the input.

Actually, the only setting available in BIOS is the minimum allowed fan speed, which as you say make the fans really quiet. However, in real use, the fans never spin that slow, not even near. Seems to be a common problem with this model. Well, I have 3 Fanmates coming my way, so I hope that will sort things out.

The stress test is a good idea. Will try the two options. Was just curious if there is any theory about these things. :)


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 Post subject: Re: Small fan blocking or helping big fan?
PostPosted: Mon Dec 27, 2010 3:17 pm 
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Joined: Thu Feb 10, 2005 11:20 am
Posts: 2175
Location: TN, USA
Since the shroud doesn't cover the entire length between the two fans if the 70mm isn't blowing as fast the extra air will blow around it and help cool other components in the case. The 70mm fan may also continue to work if the larger fan fails giving you some redundancy for failure modes.

If the shroud were complete then yes you could be right in being concerned about it blocking airflow.

See the Mugen-2 review for an example of a case where 2 fans on each side of a heatsink can improve temps.
Quote:
Where the Mugen-2 really shines is when a second quiet fan is added in a push-pull configuration. With both fans set to spin ~700rpm or lower, there is essentially no noise increase compared to just one fan, but the cooling performance moves up to levels never achieved in the past.


your testing will have to be thorough (not just one or two load levels) to be sure how it behaves real world.

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 Post subject: Re: Small fan blocking or helping big fan?
PostPosted: Wed Jul 18, 2012 5:39 am 
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Joined: Tue Jul 17, 2012 11:03 pm
Posts: 1
xi

What was your final solution? I am about to try do the same with a DC7800 SFF and your experience would save me time and money. Than you.

BTW, how did you remove the CPU fan without the bios reporting that is missing? Did you connect the rpm reading from the artic? If that is the case how?


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 Post subject: Re: Small fan blocking or helping big fan?
PostPosted: Sun Jul 22, 2012 6:30 am 
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Joined: Thu Feb 04, 2010 6:06 pm
Posts: 3395
Location: US
spcrreader wrote:
xi What was your final solution? I am about to try do the same with a DC7800 SFF and your experience would save me time and money. Than you.
washu wrote:
Either way just run some stress tests with and without the 70 and check your CPU and GPU temps. If they are too high without the 70 then put it back.
While that was good advice, it was highly likely that the small fan was contributing to cooling, not detracting from it.

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