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Samsung/Joule Gen6
Welcome to the forums @isaac. What you’re describing is really common on systems that weren’t flushed properly at installation. We went through something very similar with our own heat pump, and the good news is that if you keep on top of cleaning the strainer, the issue usually settles down over time.
From the photo, it looks like you’ve got a mix of general sludge and early-stage corrosion. The sludge/gunk will clear with repeated cleaning (in our case it took around a dozen cleans before it finally calmed down) but any corrosion you’re seeing won’t go away on its own.
E911 on a Samsung is (from memory) a flow-related error, and it’s extremely common when the strainer starts to clog. If it’s reappearing after three or four days, I’d honestly be cleaning the filter once or twice a day for a little while just to get ahead of it.
Remember to keep an eye on your expansion vessel pressure.
Once the biological/algae-like sludge has stopped appearing, it’s worth getting a proper inhibitor/biocide into the system. This Fernox one works well with low-temperature systems: https://amzn.to/3MiBpr4
You can add it through a towel rail. There’s a strong chance your original installers didn’t dose the system at all, so get a bottle in now and then ask your installer to check the levels properly at your next service.
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