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NBP too big?

dvnyc Member Posts: 2
As the often discussed NBP issues are from being undersized. What about being oversized? I need to redo the NBP for a weil mclain 378 boiler. It's a two pipe system, with 2 2.5" mains. The boiler can take 1 or 2 risers up to 4". The manufacturer calls for a min setup of one 3" riser and 3" header. I have access to all the to all the 4" pipe, fittings and a threading machine for free. Any fittings than 4" I would have to buy. Doing a single 4" riser into a 4" header seems fine to me. But what about 2 x 4" risers into a 4" header? What about the two take-offs? I plan the first 12" or so to be 4" pipe before reducing, but how far can I go with 4" pipe before it's counterproductive?


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,852
    NBP can not be too big. within reason Think of it as more space inside the boiler for the steam to leave the water as dry steam. Basically, the old boilers had a big area above the water line for the steam to leave the boiler into the header and the mains without a bunch of water flying along with it.

    The new smaller boilers don't have all that much room for the steam, so the manufacturer depends on the installer to understand this and create a large enough space in the NBP to simulate the large steam chest of the olden days.

    As long as you follow the "Minimums" in your design, the larger diameter pipe will never hurt the system and can actually make it better if you are up against a shortcoming elsewhere.


    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • ethicalpaul
    ethicalpaul Member Posts: 5,704
    Are you sure it's just the sizing? Too often we see the layout is insanely wrong. If you post pics of the existing piping people will offer their opinions :)

    There are great documents by Ted Parreco here on this site that shows the steam velocity for many different boilers, but I'm not sure about that one. But regardless it will give you some good information on the subject. See https://heatinghelp.com/systems-help-center/steam-velocity-table-weil-mclain-gas-boilers/

    Also see this video where I tested the manufacturer recommendations (although on a very small boiler): https://youtu.be/4IymyZB4wlI (my conclusion is that manufacturers want the boiler install to succeed and so they will not set you up to fail by specifying too-small pipes.)
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    There's no harm to oversize piping (at least up to the point of insanity...) -- provided two things: first, the fundamental geometry is right and second, insulate it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England