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Help With Main Vents

I would appreciate your help on this...

I have a one pipe steam with 50' insulated 2" header with a single Bigmouth vent. My boiler is oversized and short cycles. I have reduced the short cycling with a time delay that keeps the boiler shut down for approx 2 mins after the pressure drops to 0. The boiler cycles 8-9 times/hr but can be adjusted down by increasing the delay on cut-in.

My problem is the BigMouth opens during the initial heating cycle but never opens until the boiler shuts down on temp. As a result the largest Sunrad in the loop is doing the breathing with a very loud hiss during each cycle on pressure as well as the final time on temp. The BigMouth is open when the boiler starts a new heating cycle triggered by temp. At start up, the Sunrad is quiet.

I decided to add more venting based on suggestions from others on the Wall with a second main vent in an antler fashion. But before doing so, I installed a B&G Model 75 float type vent in place of the BigMouth to see if it would open at a higher temp. It doesn't. I think the temp at the vent is still too high for either type of vent to open and vent the main despite the boiler pressure being 0 on a 0-5PSI gauge. So it appears the additional vent may not solve the the radiator hiss.

The large Sunrad has a #6 Maid O Mist vent. I have tried the slower #5 vent but no improvement. An even smaller orifice merely slows down the radiator from heating. The vent height is probably too low but there is very little height available to install the vent(s) on a taller pipe. Is there a way to pipe this main vent in a better way?


  • Fred
    Fred Member Posts: 8,542
    First off, that's not a B&G #75. That's a Hoffman #75. Put the Bigmouth back on there. It has the venting capacity of about 5 of those Hoffmans.
    Secondly, you misunderstand the function of main vents and radiator vents. They are designed to be open to allow air to be pushed out of the mains and radiators to allow steam to enter quickly. Once the vents get hot, they will stay closed until they cool enough to open again. For mains, that is typically a few seconds to a couple minutes after a heating cycle so they are ready to allow air to be pushed out on the next heating cycle.
    What pressure are you seeing on the 0-5 PSI gauge when the system is running? Are you 100% sure the pigtail (looped pipe the Pressuretrol is mounted on) is clean and open? If it is clogged (a common problem if not cleaned annually) the Pressuretrol can't see the system pressure accurately and may let the pressure build until it blows radiator vents open, during a heating cycle. Clean the pigtail and, if you have the gray box Pressuretrol, set the Pressuretrol to .5 Cut-In and the Cut-out to "1". If you have a Pressuretrol with a clear plastic front, set the Main to 1.5PSI and the Differential to "1".
  • ethicalpaul
    ethicalpaul Member Posts: 5,628
    edited February 2020
    I think I had the same problem but it went away when I took my big mouth out of service due to leaking steam. I think it stays sealed a lot longer than a Gorton, for example. So I would try a MoM or Gorton #1 on your antler, added to the Big mouth. It will act as a vacuum break.

    Edit: I thought more about your post. I really like your time delay on pressure, but why only 2 minutes? Why not give the radiators some time to...radiate...some of that heat off? Two minutes is just enough time to lose pressure, so if you're going to lose pressure anyway, and presumably you set this up because you don't like cycling on pressure, why not really use the delay?
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Jamie Hall
    Jamie Hall Member Posts: 22,972
    A comment on timers vs. pressure cycling. You either need to have your off portion of the cycle short enough so that air doesn't reenter the system -- or long enough so that the vents can open to get rid of the air that will enter the system. Take your pick. For a boiler which is very much oversize, the latter option is often preferable -- but that will mean an 8 to 10 minute off time (more if the vents only reopen at a low temperature, like a Big Mouth), and it is possible -- depending on the amount of radiation in the building, how much too much oversize the boiler is (and how good the main venting), and how cold it is outside -- that that may not give you enough heat to keep the building warm. On a truly oversized boiler, though -- one where if one is using pressure cycling the on time is about the same or even less than the off time -- this may be the only thing to do.

    However, if the boiler on time, using pressure, is significantly longer than the off time, my opinion (others differ) is to restart the boiler as soon as the pressure drops -- or the post purge/pre purge sequence lets you (that's often longer)-- to keep the boiler nearly on a boil and all the pipes nice and hot.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • PhilKulkarni
    PhilKulkarni Member Posts: 70
    Thank you all.

    To@Fred: I checked the pigtail to both the trol and the gauge. Both moderately clean so I cleaned them again. TI cleaned them in October last year but then the boiler/pipe/radiator rust took a few days of blowdowns before the water was pretty clear. The pressure is 2.25PSI at cutout. The min diff setting in the trol is 1.

    To@ethicalPaul: The Big Mouth doesn't leak it just wasn't opening while the boiler cycled. But my explosively breathing radiator in the living room has been tamed by increasing the time delay to 9.25mins from 2. It has been 2 days and everything is quiet. I may increase the delay a bit more to between 9-15mins if it doesn't increase the time to get to 70 from a setback of 2 degrees at night. The cph has dropped.

    To@ Jamie Hall: The boiler on time on pressure was a tad longer than the off time. Since I could not reduce the cut in below 1.25psi with my older trol I had set the off time 'arbitrarily' to a few seconds after the pressure dropped to 0 for a total of 2mins.

    I will be replacing the Hoffman 75 with a Big Mouth to increase venting as suggested. With the 9.25m delay the H75 opens after 3.8mins and the BM after 4.25mins( from cut in). Perhaps the 27sec difference is not significant if the boiler was not oversized in the first place?

    Finally, because the timer is set to break on make(cut in contact closure), the actual cut in setting is trivial because I can achieve the desired time delay no matter what the cut in pressure is with dip switches. In fact I can ignore the cut in entirely and instead set the timer to make on break (on cut out contact open) and set the delay to the same 9.25mins interval.

    Many thanks to all for your expert help.