I agree that the specks would show up everwhere on the hot side if it was the heater.
I have been living in my new home for two years. I have two separate furnaces (wood and oil) connected together. In the summer when I use the electric hot water heater I have no problems, but in the winter when I use the wood furnace to heat my water I get Black specks in my bathtub. They look like iron filings but they are not as I checked with a magnet and they are not attracted to the magnet. They do not smear. They are hard specks like gravel, but they all look black . I only get these specks in the bathtub hot water tap. They do not appear at the kitchen sink nor the bathroom sink. I removed the aerators to check at these sinks and found no black specks in them. In fact , I believe that if I got these same black spots in the aerators and filters they would be all clogged up and I couldn't get any water at all. If this is gravel coming from my drilled well, why don't I get this gravel at all the sinks? If it is the tap itself that is defective in the bathtub, why don't I get these black specks when I use the electric hot water heater? I am stumped and so is the furnace man who came to check to see what could be the problem. Can anybody out there explain to me what could be causing this? Any information would be appreciated.
What is the water heating coil in the wood burner made of? It could be creating those specks, but they could be small enough to pass through the aerator and down the drain. The tub holds the water long enough for them to settle out of suspension so they would show up in the bottom of the tub once the water drained out. Iron oxide particles should not be magnetic.
"Iron rusts from Fe to Fe2O3 Ferric Oxide. Red Rust is not ferromagnetic (what we typically mean with the term "Magnetic") . Ferrous Oxide FeO (black rust) IS magnetic. "