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measuring mains to determine vent size

McKern_2 Member Posts: 43
We need to replace the main vents, which are on the ends of the 4 returns, and I don't know what to measure.

This is a 3 story, 6 unit, condo building. The boiler is located slightly off the center line, toward the back of the building

The supply mains take off the header and into the basement ceiling then drop below the ceiling about ~15 feet from the boiler (pictures attached). Do I measure the length of the supply mains below the ceiling, include the 15 feet to the boiler, and the return pipes for each vent? Also, how do I determine the diameter of the pipes encased in asbestos?

Thank you


  • Brad White_191
    Brad White_191 Member Posts: 252
    Measure the Piping above the Waterline

    Measure the length of each "steam-only" pipe by size, starting from the boiler to the end of each run.

    For example, starting from the boiler it might read like this:

    Boiler: 18 feet @4"

    Branch 1: 32 feet @3", 24 feet @2.5", 40 feet @2"

    Branch 2: 40 feet @3", 27 feet @2.5", 40 feet @2"

    Each branch would have the volume calculated (there are charts for this). Then, what I would do is split/apportion the common boiler piping to each so that each branch helps in venting that.

    Best tools for you, all available in the on-line store:

    1) Pocket Rocket pipe caliper. Get the larger metal one, not the plastic one, for what you are doing. Worth it and fun at parties. :)

    2) A copy of Gerry Gill and Steve Pajek's "Balancing Steam Systems using a Venting Capacity Chart". Proceeds go to a good cause. Invaluable booklet and so little money for what it is worth. It is an e-book in PDF format and is e-mailed to you.

    3) A copy of Patrick Linhardts, "Linhardt's Field Guide to Steam Heating". LOTS of tables, great information on pipe capacities, venting and how-to.

    4) A copy of Dan's "The Lost Art of Steam Heating" (TLAOSH). Not specific to the task at hand, but always an excellent and recurring reference.

    Unless you poke a hole to measure the depth of the insulation, I would leave it alone use a default of 1" thickness, subtracting that from the OD you measure with the Pocket Rocket.

    The worst thing that happens is that you assume larger pipes than you really have, so your vents will vent that much faster.

    Now, to be clear, we are talking main vents here. The upfeeding risers and branches are handled by the radiator vents. Radiator vents just vent the radiator they serve plus the piping feeding that radiator. If a number of radiators are served off of a common riser, allow each to share the load, with the top radiator usually having higher venting capacity.

    Some main tops have their own vents, for they themselves are mains or might as well be.
  • McKern_2
    McKern_2 Member Posts: 43

    Thank you for the info/instructions.

    I have Dan's book and I've already recommended that the Condo Board get the Venting Capacity Chart--I'll add Linhardt's book to the list.

    Re the Pocket Rocket pipe caliper--I guess I'll just buy it myself--sounds like too much fun to share with the Board.

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