I believe there is issue with water and yes there is build up but mostly with hot water lines, Water has been tested and chloride, calcium, magnesium have been at the higher levels but with in manufacturers specs. My water is hauled in and I use 2 different wells both are 10 miles apart but test close to the same. Other water sources are almost 50 miles away and have not been used. I been told treating the water creates another problem. Everyone in my area over 100 homes uses the same water and don’t have issues, but I seem to be the only one with a stainless steel water heater.
Original same model water heater that was installed in 2001 lasted 11 years new ones only get 10 to 11 months if lucky and even had one fail have a catastrophic leak 4 months.
I have lower the temp to the thermostat to 130 degrees was told unit easily can withstand 150 to 160 degrees for radiant floor but have since lowered temp and found it pretty much does the same job.
Lowered temperature,installed anode rod and will shut unit down when not in use to see if it helps. If not glass lined unit will be next and will get better information about water treatment.