In this excerpt from his Classic Hydronics seminar, Dan Holohan teaches us about the roots of modern hot water heating. At the turn of the 19th Century, central heating was new and split into two camps: those who favored steam at low pressure (what they called vapor heating), and those who preferred gravity hot water because it seemed safer. The gravity systems, up to that time, were all two-pipe systems, and the pipes had to be large to keep the frictional resistance presented to the rising hot water low. This meant that hot water systems were more expensive than steam systems, which had large supply pipes, but small return pipes, and were generally easier to pipe.

Oliver Schlemmer comes up with the O.S. fitting just as we enter the 20th Century and that introduces one-pipe hot water heating for the first time. In the O.S. fitting, we see the roots of the diverter tee, as well as inklings of primary-secondary piping, which will follow 50 or so years later.

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