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Steaming up the boiler room

I have a 2 pipe system with steam traps at each rad. There's a condensate tank and pump at the boiler. There's also 2 - 3/4" open ended pipes on the returns before they enter the tank. So these are on the house side of the tank and pump. All of a sudden, after decades of operating fine, we're getting steam out of these 2 pipes. There is a check valve between the pump and the boiler. I do not believe that to be the issue. There is a steam trap at the tank that is on the line coming off the end of the steam main in the basement. I replaced the cage unit in it. Didn't solve the issue. Today we replaced the cage units in all the radiators that do not have Danfoss TRV's on them. We then turned off the TRV's taking those rads out of the equation. Same as before, 45 minutes after starting up the boiler steam is coming out of the open pipes. I'm missing something but darned if I see what it is.

Only other thing that has changed is that I was originally called out because of a steam pipe leaking in a brick wall. We opened the wall up and discovered a broken CI fitting. It was the return of a radiator above. We capped off the supply and return to that radiator until it could be fixed. (Not going to be easy). Since those pipes are completely capped off from the system I can't see where it could be an issue but thought it prudent to mention because it is the one thing that has changed.

Any thought would be very welcome.


  • Jamie Hall
    Jamie Hall Member Posts: 22,922
    Does this application have true dry returns? And, if so, is there somewhere out there in the wilderness where there is supposed to be a loop seal between a steam main and the corresponding dry return? Often formed by a drip from a dry return and a drip from a corresponding steam main coming down to a wet return.

    If there is no wet return there and the boiler pressure is a little high, that can blow the loop seal and you'll get steam in the return. In the latter case -- with the wet return -- if the wet return isn't wet any more (someone lowered the water level somehow) you'll get the same effect.

    You can get the same effect -- even more spectacularly -- if there is a drip from a steam main to what should be a wet return which isn't wet.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,327
    Either a bad trap or what @Jamie Hall said above