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Black particles in water: Navien tankless water heater to blame?
Small black particles started appearing in our water a few weeks ago. Appears to only be the hot water. We have a Navien tankless water heater. I'm not sure the model but have attached a picture of the inside. Looks relatively new (we bought the house a year and a half ago). We had the Navien flushed/serviced. Did not fix the issue. We had the valve on the main water line into the house replaced as recommended by a plumber (in case rubber is the issue). House has copper and PVC pipes. No visual evidence of flexible/rubber lined piping in the home. We're not sure exactly what the particles are. I have attached a picture. They smear when you rub with your finger. Any ideas as to what this is and how to fix it?