on the home? if so it must be in the bypass mode :)
Sure looks like a hard water issue. testing the water is the only way to know what you have and how to properly address it.
Also the higher the temperature in the indirect the more minerals seem to settle out.
An electric water element is a good example of how the minerals stick to a hot surface.
A deposit like that REALLY cuts down on heat transfer. They must have noticed diminishing HW output?
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Take a look at the attached photos. This is from a Triangle Tube indirect heater. I believe it was from poorly treated water. It affected everything that had hot water in it, including all the hot water pipes and fixtures in the house. We changed out the unit, a plumber is dealing with the rest.