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Two pipe vapor system piping question
In my ~1915 two pipe vapor system, the steam pipe and dry (condensate) return travel round the basement together, with risers going up to radiator groups. At the end of the basement run, the steam pipe drops down to create a loop with the wet return.
My question is specifically regarding the last set of risers that go up to the second floor right at the point where the steam pipe connects back to the wet return. I would expect that there would be risers for the steam and the dry return, just like the other radiator feed piping. However, there is additionally a third riser connected to the loop that connects the steam and wet return pipes. So instead of two pipes, there are three pipes going up to this last set of radiators on the loop. Why would there be this third pipe going up?
This is a question of practical importance since I have a small leak in in one of these pipes behind a first floor wall, so when I open up the wall and locate the leak, I need to know the purpose of this third pipe. I can not see how the piping is configured at the radiator bank on the second floor since everything under wood flooring there.
Any help is much appreciated.