I took this advice from both the forum(s) and my plumber brother that I could use black iron rather than copper, (next to silver and gold on Mendeleev's table) to save considerable money in pipe and fittings when plumbing up my "new" hydronic system. Did so, but since firing up in Feb of this year we have experienced a good and continuous amount of iron oxide in our DHW. This, as I now know, will continue until I "rip" out the black iron DHW loop and replace it with copper and fittings. There are 3 - 1 1/4" ball valves, a strainer, 2 threaded dielectric unions and a bunch of other "stuff" to modify or replace. Frankly, I knew of the iron oxide formation on the interior of the black iron, but didn't know it would continuously flake off into the supply. Thinking was that it would coat the pipe's interior and then remain there. Not so! I never saw a warning in the forums not to plumb this way for DHW, so I hope this thread will advise future DIYers and professionals (if they were still unaware) that to not use iron piping and fittings in the DHW loop. My fittings and pipe will arrive by UPS today and the bill for replacement parts, not counting labor (mine) and torch fuel, solder, and flux did cost in the neighborhood of $500. Wicked good lesson on Murphy's Law.