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Every time I see this come back up to the front of the line, I laugh…. I had this debate with Paul because I learned what was in the books so long ago that it was as if that dimension "A" and the Equalizer were second nature to me. And when you know something… YOU KNOW IT!

Paul's experiment and video showed me that on a small residential system there is not much pressure drop from one end of the main to the other. That whole steam chest of the boiler pushing water out the return backwards did not need 28" of dimension "A" to force the water back into the boiler.

But if the pipes are restricted with years of sediment, and we are talking about a rather large building, then that dimension “A” can certainly help the water get back to the boiler. But when doing the testing on a small residential system, at such low pressures, that is well cared for, and probably has no sediment in any of the wet returns, then you basically have a U tube manometer with equal pressure on both ends of the main, at the boiler and past the last radiator, at the last drop into the return.

Paul finally made me change my opinion of the whole thing. Those pictures in the book are only illustrations. They did not use glass pipes and measure the pressure differences to see inside those pipes. They made educated guesses based on what they found with previous systems. Some of those systems were built before the Carbon Club members made the 1.5 PSI rule that we all use today. I might guess that 50 to 100 PSI steam might have a bit more friction traveling through some of those old steam pipes and that the dimension "A" may have been more that 28" to overcome the 2 or 3 PSI difference on those older 100 PSI systems. Ya never really know where those old beliefs, guides and rules came from, and why they are still in use today.

There is a story about how the railroad track gauge (width) is based on the Roman Chariot wheel base. As the road ways were traveled by these chariots they made groves and ruts as many carts rolled over the same path over and over. If you designed a chariot with a different wheelbase, your wheels would soon fail trying to find their way into the existing groves. If you didn't want your carts to have constant wheel failures you adopted the standard wheelbase as all the chariots. This lasted for centuries and when the railways and steam locomotives were introduced, the existing wheelbase was already an engineering standard and they had no reason to change it.

Is this truth or an old urban legend? Who knows? It is a good story to explain the odd dimension of 4' 8.5" or 143.5 cm as the standard rail gauge in the USA. Such a strange number to use. And Roman Chariots were never used in the 2 and 3 centuries in the Americas. We weren't even called the Americas yet! But you can easily fit a team of 2 horses side by side within that 4' 8.5" wheelbase so the horses didn't lose their footing in the ruts.

But now that all the tracks are 4' 8.5" wide, then all future tracks will be 4' 8.5" even if there is a better dimension for future needs. It is that way because that is the way we learned it.  That weird number is kind of like Dimension “A” in all the old engineering books.

I sort of get the feelings, without doing proper experiments, that there is one set of rules for all systems. Small residential to large commercial. Well piped, under pipe, over piped..... Take a very large commercial system, maybe under piped and with very dirty water. Good possibility there will be a significant pressure drop by the time the steam gets to the end of the main.