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Old March 25, 2015, 05:51 PM
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Default A Preliminary Study on Bottom Air Intake

Introduction:
I have been designing my own PC case, and one question that came up was: How tall does the case feet have to be if you have air intake on the bottom? The present study is conducted in order to answer this question. In the process of testing, I have also discovered some interesting things about air intake in confined spaces.

Test Setup and Methodology:

Test Configuration:

CPU: Intel i5-4590S
Cooler: Silverstone AR05
Motherboard: Asrock Z97e-itx/ac
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory
Storage: Transcend MTS400 256GB M.2-2242 Solid State Drive
Case: Mini-Box M350 HTPC Case
Power Supply: picoPSU-16​0-XT + 192​W Adapter ​Power Kit
Case Fan: SilenX EFX-08-15

This test configuration is chosen for convenience. It is my main PC. Here is what it looks like:



Both the CPU cooler and case fan are setup to intake air.

Temperature Assessment Methodology:

I assess an "idle" temperature and a "load" temperature. The idle temperature is obtained by keeping nothing but the desktop open on the computer for 10 minutes before recording temperature values. The load temperature is obtained by running a program I wrote for 10 minutes before recording temperature values.

This program was written to utilize all 4 cores, and I believe it gives a more realistic scenario of load than prime95. Another reason for choosing it is because in this system, prime95 stabilizes around 85 deg celcius, which is a higher baseline load temperature than I would like.

The program I use to take down the temperature is HWMonitor. Whenever the temperature exceeds 85 deg celcius, I terminate the test and record the time.

Methodology:

To make the test configuration intake air from the bottom, I flip it over. To figure out how tall the case feet needs to be, I incrementally prop the case up with pennies on each corner. The width of a penny is approximately 1.5mm.

I run the same temperature test for 2 scenarios. In the first scenario, I keep the test configuration as is, with all intake air holes open. In the second scenario, I use tape to cover the intake air holes that are not directly over the intake fans. See picture explanation below:

Scenario 1:


Scenario 2:


Results:

Idle temperature readings are on the left while load temperature readings are on the right.

Scenario 1:

0 penny - 0 mm case feet - Failed after 7 minutes


1 penny - 1.5 mm case feet - Failed after 5 minutes


2 pennies - 3 mm case feet - Failed after 5.5 minutes


3 pennies - 4.5 mm case feet - Reached 84 deg Celcius under load


4 pennies - 6 mm case feet - Reached 80 deg Celcius under load


5 pennies - 7.5 mm case feet - Reached 79 deg Celcius under load


6 pennies - 9 mm case feet - Reached 76 deg Celcius under load
Idle temperature unavailable because I accidentally saved over it


7 pennies - 10.5 mm case feet - Reached 75 deg Celcius under load


Not inverted - Infinite height case feet - Reached 67 deg Celcius under load


Scenario 2:

0 penny - 0 mm case feet - Failed after 4 minutes


1 penny - 1.5 mm case feet - Failed after 3.5 minutes


2 pennies - 3 mm case feet - Failed after 6.5 minutes


3 pennies - 4.5 mm case feet - Reached 82 deg Celcius under load


4 pennies - 6 mm case feet - Reached 77 deg Celcius under load


5 pennies - 7.5 mm case feet - Reached 71 deg Celcius under load


6 pennies - 9 mm case feet - Reached 68 deg Celcius under load


7 pennies - 10.5 mm case feet - Reached 67 deg Celcius under load


Not inverted - Infinite height case feet - Reached 66 deg Celcius under load



Discussion and Conclusion:

To me, the most interesting thing about the test results is that Scenario 2 performed better than Scenario 1. This is contrary to my intuition, which suggests that Scenario 1 should perform better because it has more air holes at the intake, requiring less effort on the part of the fans to draw in cool air.

The mystery is solved when I touched the intake panel. In Scenario 1, the intake panel is always warm while in Scenario 2, the intake panel is always cool. This suggests that hot air is circulating at the intake panel during Scenario 1, creating a feedback loop of heat. I believe that what happened was that cool air is drawn in by the fans, heated by the CPU, then pushed out the side AND out the holes on the intake panels that are not directly over the fans.

Here, we arrive at the first conclusion that may be drawn from this study:


When intaking air from the bottom of the case, having more air holes is not necessarily better. In fact, it is best if there are no air holes except directly over case fans.

This conclusion has a few implications:

1) For case designers, you should only put air holes directly over the case fans for areas that will intake air from the bottom.

2) For builders, if you have a case that has intake air holes on the bottom for 2 fans, you should:
a) Use 2 fans there.
b) Don't use any fans there
c) Use 1 fan and block the other opening somehow.
Using only 1 fan there without blocking the other opening may cause hot air recirculation.

So now we go back to the original question: How tall do case feet have to be when you have intake holes on the bottom? The results from Scenario 2 brings us to our second conclusion:


Assuming hot air recirculation does not occur, the case feet should be about 9 mm or taller in order to achieve approximately the same level of cooling as open air.

I would recommend using case feet 10 mm or taller to be safe.


Future Work:

There are a few sources of error in this study. First of all, the temperature control in my house is not perfectly stable, oscillating by a few degrees celcius. This may account for some variances in the results. Second of all, each Scenario was only tested once, meaning that results may be skewed by random cold-air-pockets in my house. Repeated testing and averaging of the results should be done to minimize this possibility.

The testing is by no means comprehensive, and further testing needs to be done to better characterize the effects of hot air recirculation at the bottom of the case. For one, using a thicker PC case may reduce the hot air recirculation effect, since the air brought into the case needs to travel a longer distance before bouncing off a wall and returning to the bottom air holes. Furthermore, this study only addresses the temperature of the CPU, but does not test the effects of hot air recirculation on GPU intake, PSU intake, nor general internal temperature of the case (the temperature sensed by the motherboard). A follow-up study should be done to address these specific scenarios.


Thank you for reading, and I hope that this has been informative!
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Old March 25, 2015, 06:34 PM
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Thanks for the info and the good work. I love getting solid information like this. Let us know your results if you pursue it any further.
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Old March 25, 2015, 10:01 PM
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Thanks for this. Great test!
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Old March 25, 2015, 11:08 PM
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A nice simple test that I'm sure will answer some peoples questions. Thanks for taking the time to test and share your results!
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Old March 26, 2015, 07:30 AM
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Well done comrade!

This supports my experience with cooling PCs. It's not how many case openings and fans you have, it's how you channel your air-flow.
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Old March 26, 2015, 07:56 AM
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Thanks everyone! Any suggestions for future tests?
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