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2-pipe steam air vent system not heating

Discussion moved from another Forum
I have a customer with a 2 pipe steam vent system that is not heating. We replaced her boiler that had a leaking heat exchanger and had not ran in over a year. The customer said before the boiler died the system worked well. The steam does not go any further that the end of the supply main. There are no traps on the system. Its a dry return with a main vent near the boiler. There are vents on every radiator and at the end of the supply main. the supply grades down from the boiler until half way were it changes to sloping up. At this slope grade change there is a tee for condensate to return directly back to the boiler. My guess is that the drip return on the supply is clogged and there is water blocking the steam from travelling down the supply???? The boiler cuts out on pressure and the steam never travels past the horizontal supply main. There are 2 radiators that are supplied before the grade change that do get hot. I am attaching before and after pics of the boiler itself.

Dan Holohan Mod > Silas • 37 minutes ago
That could be, Silas. Can you show me some pics of a radiator supply and return, and a wide shot of the radiator with the pipes? You might also want to move the conversation over to The Wall to get even more thoughts from the other guys.

Thanks for the quick response. I won't have access to get more pics until after the new year, but I can try to describe. The radiators have a 3/4" supply with a valve attached to the top of the radiator and a 1/2" return pipe out the bottom with no valve. The air vent is located half way up the side.


  • Abracadabra
    Abracadabra Member Posts: 1,948
    Was the boiler properly skimmed after installation?
  • Jamie Hall
    Jamie Hall Member Posts: 18,903
    Check your pressure. If that low point on the return is getting flooded -- either from pressure build up backing water up from the return or from a clog -- that could do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Steamhead
    Steamhead Member Posts: 15,329
    Something doesn't add up. If there is a vent on the dry return, there should not be vents on the radiators. The radiator piping pattern you describe is that of a Vapor system that vents air at a central location, not at each radiator.

    Post some pictures of some radiators, the dry return vent, what happens at the ends of the steam mains etc.

    Where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Jamie Hall
    Jamie Hall Member Posts: 18,903
    I was making a faulty assumption... that the system hadn't been really seriously knuckleheaded. I could well be wrong! Got to admit that the valve arrangement on the radiators does sound like it had been a vapour system at one time...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England