I have just recently got a Raspberry PI 4B with 8GB RAM and I am wondering how much cooling is necessary.
The usecase will be installing Arch Linux and using it as my daily driver, so it probably has to be under 60 degrees Celcius.
Which cooling system should I get? IF you have any other recommendations about Raspberry I would be very happy :)
I will build a custom 3D printed case for it in the size of a small desktop computer: https://www.youtube.com/watch?v=9sb_zuHGmY4
With that OS on it I doubt it gets very warm, with the PiOS on mine it was a good 10C higher than it is now with pure Debian Buster on it. The load has gone from a constant 1+ for a headless server to just about nothing like it should be. Mine is in tiny little case just big enough to contain it with a 1" fan that came with it. A decent sized case with reasonable air flow show do you even better than I get. With a desktop you will get a higher load but these things are good to 80 C before they start to throttle down the speed.
root@buster-raspi:~# vcgencmd measure_temp
temp=45.0'C
root@buster-raspi:~# uptime | cut -d ',' -f 4-6 | sed -e 's/^[ \t]*//'
load average: 0.04, 0.10, 0.09
Alternative method to get the temps without the Pi command I installed by taking it from their install of PiOS Buster 64bit, divide by 1000.
cat /sys/class/thermal/thermal_zone0/temp
46738
Cooling the RPi 4B is not necessary. I say this because the RPi has built-in thermal protection - a closed loop system that monitors the temperature sensors in the RPi hardware, and "throttles" the system clock when necessary to keep the temperature in the Safe Operating Area. AFAIK, no one has ever lost their RPi to over-heating. There are several other Q&A here that are closely related to your question - for example.
The exception to the "No Cooling Needed" advice would be if your RPi were either located in a high ambient temperature environment, or engaged in a high stress application such as mining bitcoins & that sort of thing.
RPi 4 Thermal Testing performed by the Raspberry Pi Foundation is mostly self-congratulatory publicity, but it does contain some useful information. For example: RPi will run cooler if placed in a vertical orientation! There was also independent RPi 4 Thermal Testing by Tom's Hardware, which provides a slightly better explanation of its results.
I have added heatsinks to my RPi4, and I have a case with a fan built in, but I've never found a situation where it was needed. I do monitor the temperature of my RPi4 with a small script that I run as a cron job:
crontab:0,15,30,45 * * * * /home/pi/take_temp.sh
#!/bin/bash
# shell script take_temp.sh
CURRENTDATETIME=`date +%D%t%R`
TEMP=$(</sys/class/thermal/thermal_zone0/temp)
echo $CURRENTDATETIME $((TEMP/1000)) >> /home/pi/temp_data.txt
Since I began logging this temp data on 12/30/19, the highest temperature I've recorded was 59℃. My RPi4 is powered up & running 24x7, but is lightly loaded for the most part.
If you decide to go ahead with the fan, this Q&A may be helpful - note in particular the option using the device tree overlay. Alternatively, if you are happy for the fan to run continuously, there are fans available that will run from either the 3.3V (at a slower speed) or the 5V supply (at a faster speed).
Better to add 5v fan on top of the heat sink to reduce the heat and when adding fan it is better to control the fan speed using PWM according the CPU temperature.
