I'm running 100% DP Ultra in a loop with a D5 Next, Ultitube 100, Aquacomputer 16/10 EPDM tubing, a HIghFlow NEXT, LEAKSHIELD, a Cuplex CPU block, and Kryographics XCS GPU blocks. Every component in the loop aside from the radiators and barb fittings is from Aquacomputer.

Provided that the GPUs and CPU are operating at acceptable temps, what is the maximum recommended coolant temp for continuous 24/7 usage?

I don't run any overclocks, so I find that CPU & GPU temps are just fine with coolant as hot as 50C. To me, "just fine" means there is no thermal throttling and temps are at or below what they were using the stock air cooler and fan curve (where applicable).

I currently have my target coolant temp at 38C and was just thinking that the loop fans could run slower and the room air conditioner could run less if the coolant could be 45C instead. But I also don't want to put the loop in danger by running it too hot for months at a time.

When I see coolant temps that people post on various forums and subreddits, I'm always left thinking the person is either a liar or an idiot I see claims of 25C being normal from people who are clearly lying and claims of 60C being just fine from people who have hacked together loops that make my face twitch when I look at them. So, I've never really known what temp to be shooting for.

I'm kind of hoping that someone from Aquacomputer will see this and share a number. Literally everything in my loop is from them except for the rads (they don't have a size compatible with this case, so I'm using Alphacool) and barbs (which they don't make, so I used Alphacool). If there is anyone on this planet who knows the highest safe temperature for continuous use, it is someone who works there.

Cooling efficiency is an entirely different topic. I am not asking if my loop is as efficient as others (although efficiency does go up as coolant temp rises compared to ambient).

With ambient air at 27C and the system at full ~900w thermal load, my loop maintains its target of 38C with the fans oscillating between 60-70% and pump at 90%. To maintain a target coolant temp of 50C, the fans would need to run at more like 30% and the pump at 60%. If 50C were safe for the loop, it would make sense to set that as the target temp to both reduce noise and improve longevity and even switch to lower airflow but quieter fans. I have already verified that this coolant temp does not negatively impact CPU and GPU performance. What I do not know yet is whether this temp is safe for the blocks, highflow next sensor, pump, tubing, and seals.

I'm curious if 50C is the 24/7 operational limit, or if 50C is the limit for occasional accidental exposure. If I recall correctly, the default alarm setting on the D5 Next is only 40C.

Quoted from "Hufeisen"

Yes, the 50°C is the 24/7 operational limit.
The Pump itself is rated for 60°C, however, the electronics module of the D5 Next can get overheated if the water temperature rises above 50°C.

Perfect! Thank you. That means I can let my loop run warmer and let my fans and pump relax a bit