Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Venting Capacity Chart

J.A. Member Posts: 18
A question about the Venting Capacity Chart. The chart gives values for 1oz 2oz and 3oz, but my system doesn't even start until 8oz (.5psi). How do I determine that new value? Multiplying the 1oz value by 8 gives me a different number than multiplying the 2oz value by 4. Am I missing something very simple here, or does everyone just go off of the 3oz?


  • Vents

    The vents only operate when there is no pressure and the piping is cold. When your system starts cold, the pipes are filled with air. The thermostat turns on the burner and steam is produced in the boiler. As steam is produced, it slowly begins to built pressure which forces the air out through the main vents. The vents, feeling the heat of the steam, then close. The pressure keeps rising until it reaches the set cut off point on your pressuretrol and the burner is turned off.  When the pressure in the boiler drops to 8oz. the burner turns on again. This cycling continues till the thermostat setting is satisfied.   The  8 oz. setting that means there is still pressured steam in the pipes which means the vents are closed and aren't operating so this isn't applicable to the 1 oz -  3 oz. table.  I always use the lowest (1 oz.) figures as this is the minimum venting and high pressure gives faster (better) venting.

    - Rod
  • haaljo
    haaljo Member Posts: 112
    System pressure starts at zero

    Your system pressure starts at zero and ramps up to 8oz, I think your gauge is not indicating pressure until 8oz but there IS  pressure. The idea is to get rid of the air in the mains A.S.A.P,  and that's when the boiler pressure is very low; when your first making steam.

    You should also be reading about running your system lower than 8 oz for energy efficiency et al, Great information on this website and Dan H.s books. First one to read is "We Got Steam Heat! "

    When you say your system starts at 8 oz that may mean the pressure continues to rise to levels that can cause vents to fail and other problems.

    The good news is you have lots of opportunities!
  • J.A.
    J.A. Member Posts: 18

    After some initial confusion on my part that was cleared up by your posts, I now understand.
This discussion has been closed.