I was first introduced to black water in 1972 as an apprentice at F. W. Behler, where I had just begun the never-ending learning curve. We were draining a hydronic hot water system that served a large row home with standing cast iron radiators. The water was dark colored and smelled a bit funky, but it was explained, to me, that this was perfectly normal and highly desirable because that indicated the hydronic water was essentially devoid of free oxygen.