We found a similar problem a few years back in a condominium complex. The building had two 80 gallon commercial hot water tanks that served the entire building. We investigated and found that although the flecks were throughout the hot water distribution system and both tanks, one of the tanks seemed to have a larger amount of the black flecks. We concluded that the tank’s glass lining was defective and the tank required replacing. We proceeded to replace the tank, thoroughly flush the system and the problem was resolved. I would suggest starting with inspecting the tanks and also as much piping as possible to guarantee it is all compatible with domestic water. Hope this helps.
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