Here’s a screenshot of my temperature sensor readouts at idle. The two fans that appear in the output are the stock 120mm 1200RPM fan that came with the CS01-HS (FAN2) and the NH-L9i (FAN1).
From the image above, it’s clear that the X550 (Onboard LAN) is the hottest component on the board but not by a very wide margin. The X550 data sheet specifies that the maximum operating temperature for the chipset is 103°C. According to that document:
Properly designed solutions provide adequate cooling to maintain the X550 case temperature (Tcase) at or below those listed in Table 14-2. The device should function properly if case temperatures are kept at or below those presented. Ideally this is accomplished by providing a low local ambient temperature airflow, and creating a minimal thermal resistance to that local ambient temperature. Heat sinks and higher airflow might be required if case temperatures exceed those listed in Table 14-2.
Personally, I have been aiming to give all the chipsets on the board a minimum of 15°C of headroom at idle so any cooling solution that keeps the X550 at or below 85°C is acceptable to me.
I could not find a comprehensive data sheet for the X570 but this thread suggests that it also has a max operating temperature of 100-105°C which implies that 49°C is very safe.
There are a few aspects of my build that may be contributing to my satisfactory temps. First, when mounting the NH-L9i I made sure to orient the fins parallel to the RAM slots, as described in the video below, in order to prevent the cooler from dumping hot air into the memory bank.
Coincidentally, this also lines them up parallel with the fins on the X570 heat sink. In the CS01-HS, this means that cool air will be pulled up from the bottom of the case by the intake fan, flow through the X570 heat sink and then up into the fins of the NH-L9i, increasing the airflow passing through its cooling elements, before finally exhausting out the top of the case. I’ve added some blue arrows to a pic of the board below to illustrate airflow. For folks who don’t have the CS01-HS, in this case the board is mounted vertically with the I/O ports on the top and the X570 chipset on the bottom near the floor.
If my airflow model is accurate, the X550 - way up in the top right tucked between the RAM and the solid I/O shield (not pictured) - is not getting much incidental exposure to moving air from the NH-L9i exhaust and is likely at least partially blocked off from the flow generated by the case fan so it doesn’t surprise me that it’s the toastiest component on the board even before considering that it is a hot piece of silicon to begin with.
I should emphasize again that the temperatures above were captured at idle and I am not pushing the system very hard at all. I’m currently running Unraid with 6 docker containers + a Plex media server in a windows VM and the X550 is only wired up to a 1GbE network (planning to upgrade to 10GbE in a few years). My network throughput at idle is a measly 50Kbps and CPU load is 1-2%. My machine also lives in a cool basement, ambient temp <20°C. If temperatures become a problem my plan is to swap the stock case fan with a Noctua NF-A12x25 – the stock fan is a little noisy so I may switch it out just for acoustics rather than thermals.