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Main vent type and sizing

I am a de facto (but novice and unpaid) building super for our small 8 unit condo building (4 floors, 2 units per floor, ea unit ~1100sq ft).  We have a one pipe parallel steam system and have a company that I generally trust to help us with its operation.  Boiler is a 650k btu Weil McClain natural gas boiler and was installed in 2015 (poorly by a company that should never be allowed near steam and we’ve been fixing issues ever since we moved in in 2017).  Operating pressure is around 1psi as best I can tell from the gauge.

We have two mains, one for each side of the building.  Shared common piping is 15’ of 4” piping.  

North loop is 71’ of 3” piping, with last 9’ before the vent (after the last riser) being 2” pipe.  Main line vent is a Hoffman 75h and was replaced about 2 years ago.  Currently no reports of venting/heat issues on north loop.

South loop is 118’ of 3” piping, with last 11’ before the vent (after the last riser) being 2” pipe.  Main line vent is a Hoffman 76 and has been there at least since 2017 when we moved in and assumed the building Mgmt role.  There are seemingly no venting/heating issues on the upper floors, but we are having several in our first floor unit on the south loop.  

The vents (Hoffman 40s) on two of the 6 rads in our unit never close and continuously vent when the boiler is running.  We’ve replaced the vents multiple times but it doesn’t fix the issue.  One of the rads in our unit isn’t heating at all.  The three rads at issue are the last ones on the south loop and these issues are all new this heating season.  I’m wondering if it is being caused by the Main line vent being bad.  I hear no air escaping the 76 vent when the boiler is heating like I do with the vent on the north loop.

So the question is could a bad mainline vent be causing the venting/no heating issues that we are having in our first floor rads?  If not, any ideas on what else it might it be?  If so, should we replace the main line with another 76 or a 75h or something else given the size/length of the piping?  From my reading on here, the 76 seems to be for a vacuum system that we don’t have (at least I don’t think that we do).

Thanks for any advice that you can provide!


    EBEBRATT-Ed Member Posts: 15,517
    I would make sure your pressure is as low as possible. They run the Empire State Bldg on 1.5 psi.

    Most recommend Gorton vents for the steam mains. You probably need bigger vents

    I would go down to the boiler room when the boiler is off between cycles or shut the boiler off. When it has cooled enough so there is no steam in the piping start the boiler up and let it run. When the steam pipe leaving the boiler is hot (can't hold you hand on it) start timing, go to the end of the main and see how long it takes for the end of the main to get as hot as the boiler supply. do this for both mains.

    You can find more information on this site "system help"-"steam" and see the attached
  • Steamhead
    Steamhead Member Posts: 16,832
    Main vents are definitely too small. Start with 5 Gorton #2 vents on the long main and 3 on the shorter one. This should get both mains hot quickly, and the steam should reach the end of both at about the same time.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
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