@raystriker
Could you recommend another cooling for the cpu instead of a water cooling? To be honest I have no experience with those, so I was looking for another cooling.
This would be my current setup right now, it is still a bit over my limit, but maybe he is willing to spend a bit more^^
https://pcpartpicker.com/list/HGDFWD
But I am not sure if the Noctua can even be used with that kind of setup?
Don't worry about it. It's a closed loop cooler (often called an aio), it's actually easier to install than a large air cooler.
Needs absolutely no maintenance. The radiator+fans bolt right up in the front of the case, and the base plate on the socket.
Watch a few videos about how to do this, and if you have prior building experience, it should all make sense very soon.
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You could also go the air cooling route.
Unfortunately, the D15S won't fit the Phanteks Eclips P400A. This leaves you with two options:
a) Opt for a smaller air cooler- The Scythe Mugen 5 Rev. B is a pretty beefy air cooler that will undoubtedly be able to handle anything you throw at it with the Ryzen 3700X.
https://pcpartpicker.com/product/8GBrxr/scythe-mugen-5-rev-b-512-cfm-cpu-cooler-scmg-5100
b)Change the pc case- The Fractal Define Meshify C is the best case for it's price class imo. It has better compatibility than the P400a, but it also is every so slightly larger. Has enough depth to house a D15s.
https://pcpartpicker.com/product/Y6Crxr
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If air cooling is a must, I'd go with the latter. The D15s will be quieter and cool better. Just be sure you can handle installing larger, heavy air coolers! One mistake and a fall on the mobo could damage the latter!
Edit: If you choose the air cooling route, I heavily recommend buying more case fans. An aio liquid cooler would have come with its own fans, so the default case fans could be repurposed for exhaust. I think the Meshify just comes with one front fan, so it'd be necessary to buy 2 more identical fans for the front (for a total of three for intake), and 2 to 3 fans of your choice for the top exhaust. Same applies for the P400a, you need more case fans to improve the case airflow to help the cpu air cooler breathe.