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Recommendation on steam main vent size


I'm inquiring to find out what size Main line vent I would need for a 1 1/2 pipe that feeds steam to a single room. This  pipe is One of two mains that feed the second story of my house.


I am getting some water leakage now from the vent on my radiator. It is a vary vent that was replace about 3 years ago. I Just tried to shut this down some.  note sure if that will help...  Clearly I have venting / condensing issue.

I am also writing to get confirmation of other issue I am having,


While renovated my basement and out of ignorance I removed the main vent when we also removed the old 2" manifold that was no longer required to heat the first floor of my house.  This had been long ago converted to hot water baseboard and was mostly capped off.

Now that the system is calling for heat I have very bad  banging of pipes. Also have an issue with water level being maintained.

On another website I received some good advise about skimming the system which I still need to do... This should, (I hope fix the water levels issues being cause by oils in the boiler) ,

Some additional information:

My system has is a split system ( baseboard hot water on first floor and steam upstairs) I believe the steam system is a "Wet" system. The boiler as two feeds coming off it,   The one arm that  I touched starts out as 2" pipe and Then I reduce it to a 1 1/2 pipe.  

The other arm of the boiler is original and untouched by me... It feeds the other 2 bedrooms upstairs. and is working fine without banging.

Thanks for you help and suggestions.


  • Rod
    Rod Posts: 2,067
    edited November 2012
    Steam Vent

    Hi- It would help if we knew a bit more about your steam system. Is this a 1 pipe or two pipe system? Pictures of your boiler and of the offending radiator would help. On the boiler take the pictures from back a ways so they include the piping attached to the boiler. We can zoom in if we need detail.

    Does the 1 1/2 main now have a main vent and if so what size is it?

    The vari-valve vents don't have a float in them to stop water coming out so you might want to switch to one that does, like a Gorton "C" or "D"  Also a Vari-valve doesn't shut off,there is only a minimum and maximum setting.

        Skimming is probably a good idea. How much up and down does the boiler water line "jump" when it is in operation? It should normally move up and down a little but a more violent action is a sign the boiler needs to be skimmed. Is the water very dirty?

    - Rod
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,849
    It depends on where it's located.

    The venting capacity of a vent depends on how much air it has to vent, how fast it needs to do it, and how much pressure it requires. Since you want to vent both main branches in the same amount of time, we need to know how long both branches are and what kind of vent you have on the 2" branch. Also, we need to know how much of the 2"–1 1/2" branch is 2" and how much is 1 1/2", and where the return drip is located. (Since you could run 1 1/2" all the way to the radiator, the best way to determine where the main ends and the take-off begins would be the location of the drip.) If you can provide these dimensions we can estimate the venting capacities.

    It would also be helpful if we could see pictures of as much of the system as you can photograph and the size of the radiator on the 2"–1 1/2" branch. There could be problems with pitch rate in the sections you replaced.

    I suspect the hammering, fluctuating water level and leaking vent are all being caused by retention of water in the 1 1/2" main.

    Is the vent on this radiator a varivalve or a vari-vent? This makes a big difference, so we need to be clear. One is made by Heat-Timer; the other by Vent-Rite. See the pictures, below.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • nvision
    nvision Member Posts: 3
    edited November 2012
    More details

    Hi, And thanks for the replies...

    Unfortunately for me, its becoming increasingly apparent that I underestimated the intricacies of steam heating. Furthermore I enclosed the pipes up into the ceilings and walls without taking any pictures but I have a good recollection of they are.

    At this point, I'm just hoping the problem with banging is mostly that I need a mainline vent and not the pitch.  I will open that section of wall to add it in but if its pitch than I'm realing in trouble cause That will not be a very easy thing to correct for at this point.

    My house is very small 18 X 26 foot print. The two mains, lets call  them( A) and (B) are roughly

    (A) ~ 16ft of 2" pipe and !8 Ft of 1 1/2in  before riser ( no mainline vent - I removed it much to my chagrin amd bad pitch)

    (B)  ~20 of 2" pipe tot he riser.  (has 1 main line vent - but behind the wall so i can't see it... I could cut this section as well .

    The registers on the second story have vari values with the slider adjuster.

    Main (A) goes into a Y from the riser - No vent.. and the Y drains to pipe that hugs the floor back to the boiler.

     From my perspective, I'm  thinking that there are several issues at hand:

    (1) My bed room with the thermastat  is not getting any heat and is probably driving the other rooms to over produce steam. This is a  situation I will fix but at the moment the steam pipe was capped.

    (2) Oil in the boiler? - i don't know but it was suggested.

    (3) missing a main line vent after last riser. Opps!

    (4) pitch - Big Opps. 

    Is there a solution to a pipe that needs more pitch? perhaps a larger vent? 

    Would it have been better to maintained the 2" pip to the riser?

    This kind of information would be helpful...

  • nvision
    nvision Member Posts: 3
    More information

    I have been experimenting,  I have turned down the thermostat so that it calls for less heat in my room (where there is no radiator presently) I think this has helped a bit.  No banging now and still some heat coming up to other radiators.  Some heat is also making its way up from 1st story zone.

    I won't be able to get you the size of the 1 existing vent until i open that section of the wall.

    I do have some other questions, are main line vents basically the same as register vents?  Is it advisable to just replace both vents on the mains? 
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