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More Fans = Hotter Computer?

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More Fans = Hotter Computer?

Postby sox 06 » Sat Nov 26, 2011 1:19 pm

I custom built myself a computer last summer. A couple of weeks ago the fan on the back of the case died on me so I replaced it. I started thinking about how I could make the computer run a bit cooler, so I decided to add an intake fan on the front of the case. However, I did not put it where its "suppose" to go (on the bottom of the case in the front by the vents). I decided to utilize the three empty 5.25" bay slots and put a 120mm fan there with special mesh bay covers. Here's my problem though:

Before adding intake fan:
System Temp: 34 degrees Celsius (about 93 degrees F)
Processor Temp: 30 degrees Celsius (about 86 degrees F)

After adding intake fan:
System Temp: 38 degrees Celsius (about 100 degrees F)
Processor Temp: 34 degrees Celsius (about 93 degrees F)

Now I know the air is suppose to come up from the bottom in the front of the case and go out the back towards the top to cool the entire system. However, those vents are still there. How can adding a fan make the computer run hotter?

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Re: More Fans = Hotter Computer?

Postby dgan » Sun Nov 27, 2011 12:59 pm

Airflow is a funny thing. Unfortunately, I've had to self-educate myself on HVAC systems because of some issues at my company office.

I'm totally guessing here since I don't know the specs, but here you go: One fan that pulls so much air per minute can only pull that much air. No more. If that fan was running at or near peak efficiency, adding another fan would not increase air flow. All it would do is add heat from having an extra motor inside the casing. So you could have a situation where you have added another 5 degrees of resistance heat, and no additional airflow.

The reason this happens is because the fans are working in tandem. Meaning one fan is pulling in, the other blowing out, but along the same ventilation path. The only way to increase airflow would be if a) the original fan was not working at peak efficiency and needed the help of another fan to pull it's capacity of air through, or b) you put in another exhaust fan to add to the capacity of air being moved.

Think of it like this. (+ = fan) If you have:

+ -------- + , the fans work together along the same ventilation line. They can't move twice as much air, they can only help each other (if needed) to move their max capacity of air. If one fan could do the work, the second fan only adds heat from its motor.

--------- +
--------- +, in this scenario, you have two fans, both exhaust, pulling two different ventilation lines, hopefully close to capacity. That is the way you double the airflow.

So my guess is your first fan was doing a great job and didn't need any help. Remove the second fan, and you'll go back to where you were before. The only way to increase airflow would be to replace the exhaust fan with one that can move more cfm, or add a second fan adjacent to it, and see if you can pull more air out of the unit that way. (The problem with that may be that limited space inside the unit decreases airflow, so again, a second fan may not find enough air to move to offset the heat it is creating.)

This is why server rooms need to be cooled. There is only so much air you can move...eventually, you have to stop trying to move more air, and simply cool the air that is being moved instead.

Hope that helps!
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Re: More Fans = Hotter Computer?

Postby sox 06 » Sun Nov 27, 2011 1:15 pm

Thanks for the info dgan ;-D

I guess what I was trying to do was bring more air into the case and not necessarily pull more air out by adding the intake fan. I kind of figured I would have a problem because the intake fan is 120mm and the exhaust fan is 80mm. I can't put a bigger or another exhaust fan in though because of how the case is designed.

Going along your two different ventilation lines theory, would two intake fans and no exhaust fan work? There is a spot on the bottom of the front part of the case for an 80mm fan. I could move the exhaust fan there and leave the grate where the exhaust fan is now for air to escape. However, I am really not comfortable with not having an exhaust fan.
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Re: More Fans = Hotter Computer?

Postby knapplc » Mon Nov 28, 2011 1:41 pm

Dumb question, but figured I'd ask to be sure - you did check that the intake fan is blowing in and not out, right?
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Re: More Fans = Hotter Computer?

Postby sox 06 » Mon Nov 28, 2011 10:18 pm

knapplc wrote:Dumb question, but figured I'd ask to be sure - you did check that the intake fan is blowing in and not out, right?

I checked and rechecked several times to make sure.

I did come up with a combination that made the system run cooler. I kept the intake fan and moved the exhaust fan to the very bottom of the case. There's no spot to put a fan there so I made one. There are four empty expansion slots below the video card, so I removed the covers and found a way to mount the exhaust fan there. This way I have the two levels of air flow as mentioned by dgan. Here are the results:

System Temp: 30 degrees Celsius (about 86 degrees F)
Processor Temp: 28 degrees Celsius (about 82 degrees F)

Compared to when I just had the exhaust fan, the system is running about 7 degrees F cooler and the processor about 4 degrees F cooler. It doesn't seem like much but I think any reduction of heat is a good thing when it comes to computers.
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Re: More Fans = Hotter Computer?

Postby dgan » Tue Nov 29, 2011 10:19 am

sox 06 wrote:
knapplc wrote:Dumb question, but figured I'd ask to be sure - you did check that the intake fan is blowing in and not out, right?

I checked and rechecked several times to make sure.

I did come up with a combination that made the system run cooler. I kept the intake fan and moved the exhaust fan to the very bottom of the case. There's no spot to put a fan there so I made one. There are four empty expansion slots below the video card, so I removed the covers and found a way to mount the exhaust fan there. This way I have the two levels of air flow as mentioned by dgan. Here are the results:

System Temp: 30 degrees Celsius (about 86 degrees F)
Processor Temp: 28 degrees Celsius (about 82 degrees F)

Compared to when I just had the exhaust fan, the system is running about 7 degrees F cooler and the processor about 4 degrees F cooler. It doesn't seem like much but I think any reduction of heat is a good thing when it comes to computers.


Good call, that makes sense. Ideally, you'd always prefer to draw (exhaust) air rather than blow (induct) air, but if that's not an option, it should accomplish pretty much the same thing.

The only thing I'd recommend is to maybe pull the case off and airdust the components more often than you usually would. When you induct, you are creating higher pressure inside the cabinet than outside, so the air is forced out the back of the computer to equalize the pressure. This means that inside the cabinet, the airflow is pretty haphazard - not bad for cooling, but will get dust in some abnormally strange places.

When you exhaust, you're creating a "vacuum" (or negative pressure) inside the case, so the air from outside is streamed in to balance the pressure. So the air tends to stream through in a more orderly fashion.
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