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A Preliminary Study on Bottom Air Intake

E

esplin2966

Limp Gawd
Joined
Jan 18, 2015
Messages
215
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:

MrdBvjK.jpg


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:
aBjwOzo.jpg


Scenario 2:
k6JzygO.jpg


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
5PLPh9p.png
c19s4Bs.png

1 penny - 1.5 mm case feet - Failed after 5 minutes
DmnyXhK.png
lukbNOc.png

2 pennies - 3 mm case feet - Failed after 5.5 minutes
SF3MyYK.png
u9MNFOh.png

3 pennies - 4.5 mm case feet - Reached 84 deg Celcius under load
CtIMh1m.png
xkYWjLR.png

4 pennies - 6 mm case feet - Reached 80 deg Celcius under load
nUUb0sI.png
MwUnyVy.png

5 pennies - 7.5 mm case feet - Reached 79 deg Celcius under load
N1IMPcs.png
Mq6PYHi.png

6 pennies - 9 mm case feet - Reached 76 deg Celcius under load
Idle temperature unavailable because I accidentally saved over it
AxX6PTS.png

7 pennies - 10.5 mm case feet - Reached 75 deg Celcius under load
UOHOZKa.png
2GE9nZy.png

Not inverted - Infinite height case feet - Reached 67 deg Celcius under load
L5FA8HE.png
p5lUiys.png

Scenario 2:

0 penny - 0 mm case feet - Failed after 4 minutes
uqhPYaJ.png
mCX4hJt.png

1 penny - 1.5 mm case feet - Failed after 3.5 minutes
DqgbXgv.png
mv1X1Gb.png

2 pennies - 3 mm case feet - Failed after 6.5 minutes
DionwIo.png
L4jdYp6.png

3 pennies - 4.5 mm case feet - Reached 82 deg Celcius under load
Gt4ILnu.png
ft2YM2s.png

4 pennies - 6 mm case feet - Reached 77 deg Celcius under load
7O7Cv5p.png
b0izmlR.png

5 pennies - 7.5 mm case feet - Reached 71 deg Celcius under load
gACpvC3.png
JmQOOwP.png

6 pennies - 9 mm case feet - Reached 68 deg Celcius under load
p5y8OeA.png
EanMNOi.png

7 pennies - 10.5 mm case feet - Reached 67 deg Celcius under load
vvw7FWh.png
2SGqDwN.png

Not inverted - Infinite height case feet - Reached 66 deg Celcius under load
Mw8iFMP.png
cr8dbx6.png


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!
 
Last edited:
Thanks for doing this!

Trying to maintain ambient temperature precisely is futile, better to measure ambient and calculate the delta.

I'd be interested in a test with the case like 20mm off the ground or something for scenario 1 to double check whether the large difference in temp is actually from the additional room or because of the orientation.
 
Aibohphobia said:
Thanks for doing this!

Trying to maintain ambient temperature precisely is futile, better to measure ambient and calculate the delta.

I'd be interested in a test with the case like 20mm off the ground or something for scenario 1 to double check whether the large difference in temp is actually from the additional room or because of the orientation.
Click to expand...

Yeah... I wish I own a lab where I can just literally keep ambient constant haha.

I was planning to go further with the feet heights, but I ran out of pennies lol. The cooling configuration of my PC has changed now, so a follow-up study will be a bit different, but hopefully the results will be the same.
 
This is going to be beneficial for a lot of people. Thanks for the enlightenment.
 
Thank you for this. Interesting read to the morning coffee:)

One thing is air recirculating inside a case, but air recycling out of the bottom, and then back in again, I did not expect. I have made ducts for the GPU fans on my M1. After this I will diffidently try to block of all of the venting around the ducts to see if there is a temp improvement... whenever I get around to it.
 
Amazing work esplin! Scenario 2 makes sense because it prevents the air from being displaced back outside the case and directly into the fan to be recirculated in that small space. Since the air can't really go anywhere else, it takes the easiest way out - the big vents.

Does that case have side vents? Maybe you could do some tests with the fan facing up (infinite feet) but in pull configuration? Or make some intake ducts pointing to the side forcing air to be pulled in from the taped up vents and out the fan vent.

It might not be lab conditions but it's definitely a real world scenario in an average setting, do its good enough for me.
 
esplin2966 said:
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.​
Click to expand...
d) Use a fan duct! This prevents the 'free air' recirculation, with a (likely minor) benefit in reduced intake restriction for the fan.
 
fusionxr said:
Amazing work esplin! Scenario 2 makes sense because it prevents the air from being displaced back outside the case and directly into the fan to be recirculated in that small space. Since the air can't really go anywhere else, it takes the easiest way out - the big vents.

Does that case have side vents? Maybe you could do some tests with the fan facing up (infinite feet) but in pull configuration? Or make some intake ducts pointing to the side forcing air to be pulled in from the taped up vents and out the fan vent.

It might not be lab conditions but it's definitely a real world scenario in an average setting, do its good enough for me.
Click to expand...

Thanks! Yeah, the m350 has vents on all sides of the case except the the side with the IO shield. I have done some quick tests in pull configuration when I first put together the build, and it did a little worse than push configuration. It is worth a test to see if it'll do better when it's taped up.

Another thing I found that's not documented is that in such a small space like the m350, it is better to have 2 fans both in push configuration as opposed to a push pull configuration, which was really interesting to me. Again, maybe this will change if I tape things up :p
 
EdZ said:
d) Use a fan duct! This prevents the 'free air' recirculation, with a (likely minor) benefit in reduced intake restriction for the fan.
Click to expand...

With a fan duct, the air will go through the heatsink as intended, but if the case is flat the air will bounce off the motherboard and possibly back out of the intake panel. This again causes hot air recirculation.
 
esplin2966 said:
With a fan duct, the air will go through the heatsink as intended, but if the case is flat the air will bounce off the motherboard and possibly back out of the intake panel. This again causes hot air recirculation.
Click to expand...
Only if the duct does not cover the entire vent.

Quick and dirty graph of temperature:
 
I'm curious about which way the heatsink is positioned because I think the direction the fins/channels are facing might impact airflow. If the fins are running left to right that might direct the airflow out the sides.

Do you have any shots of the internal layout?
 
fusionxr said:
I'm curious about which way the heatsink is positioned because I think the direction the fins/channels are facing might impact airflow. If the fins are running left to right that might direct the airflow out the sides.

Do you have any shots of the internal layout?
Click to expand...

I didn't take pictures of the internal layout, and I changed the internal layout so I can't take pictures anymore (lol). However, I know that the fins are running top to bottom.

You have brought up an interesting thing to test, which is how the bottom air intake test results are affected by the directions of the heatsink fins. In fact, I should do a quick and simple test on how the fin directions affect cooling in a flat case like the m350.
 
That's quite interesting. I was expecting scenario 2 to perform better, but not by such a large margin.
 
iFreilicht said:
That's quite interesting. I was expecting scenario 2 to perform better, but not by such a large margin.
Click to expand...

Yeah, I was surprised too. The big difference could be due to the fact that it's a very flat case, so the airflow bounces back out almost immediately.
 
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