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Two pipe system -return main with vent off mainly sucks in during cycle

Jay4334 Member Posts: 33
I was wondering if anyone can tell me why this happening? I believ it's causing water to back up to main venting, only seems to draw water to the venting antler, then gurgles for about a minutes before you can hear the water drain back down the return pipe back to boiler. ANYONE?

Thanks in advance


  • Pumpguy
    Pumpguy Member Posts: 632
    Much depends

    on the type of 2 pipe system you have.  If yours is a gravity return type with no condensate pump, your radiators will be fitted with thermostatic traps and there will be an end of main drip trap.  There will be an air eliminator valve in the return line, downstream of these traps. 

    The condensing steam creates a vacuum due to the change in volume from steam to water.  This would cause the inward flow of air you are experiencing. 

    There are other 2 pipe arrangements that don't use traps.  Still others have been converted with inlet orifices with a view toward eliminating the need for the thermostatic traps. 

    More information and some pictures would help identifying your system, but I think I'm pretty safe in saying the inward flow of air is caused by induced vacuum from condensing steam. 

    Are you having any other problems with your system?  Is the heating even through out the building?
    Dennis Pataki. Former Service Manager and Heating Pump Product Manager for Nash Engineering Company. Phone: 1-888 853 9963
    Website: www.nashjenningspumps.com

    The first step in solving any problem is TO IDENTIFY THE PROBLEM.
  • Jay4334
    Jay4334 Member Posts: 33
    System heats house quickly and evenly

    but the back flow of water to the main vents has me confused. I recently replaced the traps on all of the radiators in the house as well as the F&T trap at the end of the main. Last week I added two Gorton #2 main vents, but I still have the issue of the water backing up into the vents themselves near the end of the cycle, which gurgles for about a minutes then you can hear it run down the return back to the boiler. I also notice a water gurgling noise in some of the radiators after a heating cycle.

    Ive attached a picture of my end of main venting & trap. If the vent is removed water will pour out of the pipe near the end of the cycle. I don't have a low pressure gauge and get no register from the standard 0-30lb gauge. I do have a bit of bouncing water in the sight glass, about an inch of fluctuation.

    Could I let the system cool, and flood the boiler to help clear and clogs in the wet return? Then drain and fill to appropriate water level? With a pressuretrol in place could the system run higher than the set PSI cut in/out? If pressure is too high for some reason, why would it cause water to back out of the boiler up the return, would that mean a main was clogged? Perhaps, now that I have all new traps I should have a vaporstat installed for and even lower pressure to prevent the boiler from backing up all this water? Any other brainstorming ideas from anyone? Thanks
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    You should not

    be getting a vacuum in either the returns or the mains, at least not with those Gortons on there -- unless they are somehow seeing steam and closing.  Since they are on the return (at least that is what it looks like from the picture) they should never see steam.

    I'm not a big fan of F&T traps.  Crossover traps and drips at the end of a steam main; F&Ts, generally no.  Are there vents on the mains as well as vents on the return?

    Or am I misinterpreting the picture?

    Also, what pressure are you running at?  There is a remote chance that you are backing water out and far enough up to get to that return, if the pressure is high enough (remember that 1 psi is 28 inches).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • BobC
    BobC Member Posts: 5,473
    gauges fail

    It might be worth adding an auxiliary 0-3PSI gauge to the system so you know whats going on. Just make sure it's has a pigtail between it and the boiler.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    Gortons loop-sealed from the return??

    Hard to see, but it looks like a circuitous path for the air to travel to escape the return. Maybe I am not seeing what is in the picture? More pictures would help.--NBC
  • Jay4334
    Jay4334 Member Posts: 33

    There are no vents on the steam main, should there be? Still learning the steam system but wouldnt the steam push the air through the main, through the rad, trap would close when steam hits it and this would continue on down the line, pushing all the air out of the vents on main return? Wouldnt a vent on steam main be rendered useless once the steam hit it as it would close and not vent (correct?) In the picture, that is the end of the return and steam main. The antler comes off the main return for venting. The steam supply end in the hoffman 55 F&T trap, then passes through to the drop down return back tot the boiler. The main return stubs up to the venting antler, while continuing back to the drop down return. From that drop down return the boiler is about 8 ft away with a Hartford loop. The boiler is set at .5 lbs with 1 lb diff. Could that really push water back that far. Also as I mentioned before if I take one of the vents off that return antler at a cycle start up I get very little air coming out before a vacuum occurs. Never any steam, I can feel warm air if I stick my finger in the pipe but not hot and not steam.....and right before (30 secs)the cycle ends water will back up and out if I don't replace vent (or will back up

    and fill vents up with water, we're it proceeds to gurgle for about and min and then drain down the return and back to the boiler.

    I should make a diagram/file with the layout of my piping so people could reference it when I have questions. Anyway, my two pipe system runs parallel around the perimeter of my basement about 100' in total distance, where it ends in the picture above. The steam main is the insulated pipe which ends in the F&T trap then comes out other side and 90's into that drop down return which has about a 65" rise and about 10 feet of piping back the boiler. The 1" cooper line in the foreground runs parallel to the steam main as mentioned Tee's up 7inches to the venting antler shown in the picture while also continuing under the F&T trap attached to the main steam vent where it also ties into that drop down return. ---Hope that helps
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    I have been thinking about this one...

    but without really looking at the whole system, I can't say just where the vacuum may be forming -- and, more to the point, why the Gortons aren't releasing it.

    I will say that it looks as though the system has been repiped -- possibly extensively repiped -- at some point, judging by the use of copper in a dry return (which really isn't a good idea), which makes me wonder just a little about what else might have been done.  A full diagram of the system would probably help.

    As to steam main venting -- in principle an F&T at the end of the main, leading to a dry return and a drip , should act as a main vent.  It is one of the standard ways of venting mains.  Usually, though, there is more or a drop to the F&T -- and more of a drop to the dry return.  With that little difference between the elevation of the steam main and the dry return, I would have expected to see a crossover trap, with both the steam main and the dry return dripped to the wet return, and a main vent or set of main vents near the boiler.

    But you do need main venting, in addition to the dry returns.  Yes, the main is vented through the radiators and the traps into the return, but without main venting (or crossover traps, which do the same thing) the heat will be slow to get to the far end of the main, and will be uneven particularly in warmer weather.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    I might add

    that you note that the water will come out of the vent connections at the end of the cycle.

    A vacuum cannot do this.  This is a pressure problem.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England