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One-Pipe dry return or Main venting?

Hi All,

A four inch, basement cieling main carries steam about 100 feet (with riser take-offs along the way), then drops off into a dry return carrying condensate all the way back to the boiler room (dropping into a wet return).

However, there are two risers near the end of the main which feed 4 apartments (2 on 2 floors). These two risers loop a small perimeter, then turn into dry returns that connect to the 'main' dry return.

Currently, all my Main venting is done at the end of the dry return, just before the wet return. 

My question is...

I believe there is a balancing issue caused by steam trying to enter these radiators from two different directions, the risers and the steam carrying dry return. In other words, thet're pushing against one another.

If I vent only the end of the steam main and NOT the dry return, will this cause a different problem as far as getting the condensate back to the boiler?

Diagram attachment follows.

Any help is appreciated.  


  • Unusual return piping

    Usually those dry returns were kept separate until they came back as separate runs, each with a separate drop to the wet return.

    I'm not sure I see what you refer to when you say the steam will try to arrive at the radiator from 2 different directions, as I had problems blowing up your diagram. If you have returns joining above the waterline, then you will have other potential problems beside main venting issues, such as water hammering. Can you drop the ends of those orphans into a separate wet return at floor level? Depending on the EDR of the radiators served, maybe even a 1 inch pipe would suffice, with a main vent close to the drop, and a run along the floor to the main wet return.--NBC
  • gerry gill
    gerry gill Member Posts: 3,078
    It shouldn't be a problem..

    I've always had an issue (mentally) with putting the vents on the end of the dry return instead of the steam main, cause the moment the vent is put at the end of the dry return, the dry return just became steam main, did it not? but its sizing isn't right for a main. Give it a shot, as you can always plug it and put it back the way it was quite easily.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • jgs2nd
    jgs2nd Member Posts: 10
    Thanks for the helpful suggestions...

    There's no easy or economical way of running wet returns, so I'm going to go ahead and change the main vent locations and stop venting the dry return.

    I'm thinking that the current setup would not have posed an issue in the coal burning 20s, but now it's a problem because the boiler doesn't run long enough to get all that air out through radiator vents.

    I'll keep you posted and thanks for your help...