Two pipe vapor system piping question

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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.

JUGHNE

How is the air vented from the mains and the radiators?

Jamie Hall

JUGHNE said:

How is the air vented from the mains and the radiators?

Good question. There may be a crossover trap hiding in there somewhere connected directly to that apparently redundant riser.

dopey27177

Hard to answer the question without pictures, Usually a third pipe was for venting air to a dry return. The dry return went to a master venting point in the basement.

Jacob Myron

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The air is vented at a single master trap that sits above the boiler just before the dry return turns down to join the wet return at the boiler . The radiators each have thermostatic traps.

There are cross over traps throughout the basement and I am wondering now if the third pipe connects to a hidden crossover trap somewhere to help the steam get to the final radiator bank more quickly?

I may not find out until I open up some of the wall, although it would seem that burying the crossover trap would not be best practice (although I do have another crossover trap that is so close to the basement ceiling that opening it quite impossible).

Jamie Hall

More and more I'm betting on a crossover trap hidden somewhere. Wouldn't be the first time. A question: does that third pipe connect to the steam main (before the loop down) or to the dry return (that is, after the loop down). Your description wasn't clear. Either way it would work.

And whatever, you do need a crossover trap there - if there isn't one, install one.

zoom

The third pipe connects to the down leg of the loop as the steam main turns down and connects to the wet return that runs about 5 feet in parallel below it.

I am thinking a buried crossover trap somewhere seems the most likely. I would then have to assume that it is buried to provide the maximum distance and therefore impact of purging air and getting steam to the most distant radiator bank quickly.

If it is a buried crossover trap, it most likely would be under the floor on the second level just before the risers feed different radiators, five of them, some of them pretty big.

However, it is an extremely well constructed house, and burying a trap would seem like an unsound practice that the original piping installers perhaps would not do. I wonder if it might have been a later modification attempted to improve the speed of heating of that last radiator bank.

SteamCoffee

Wondering if you know what Brand of System you have? Vapor had many ways to create or enhance pressure differentials to bet steam to the right place. Traps, rad valves, boiler room stuff should yield clues, pics would be best.