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Reducing from 3/4 to 1/2

kensheets2 Member Posts: 36
I have installed a 3/4" 2 pipe reverse configuration for old cast iron radiators in a 2 story residential, completely new installation. each floor it's own loop.
question is cuz the pipe is so difficult to work with and I did not install it in a manner that would have made it easy to tee off of, would it be acceptable to tee off with 1/2" at each radiator from the 3/4 supply/return lines? also is it a issue to reduce to half inch at manifold?


  • mattmia2
    mattmia2 Member Posts: 5,533
    Notching the top/bottom of a joist is a big issue.
  • kensheets2
    kensheets2 Member Posts: 36
    yes agreed, this is 1910 construction and I sistered up 2x6 against original bowed 2x8 to just level floor and give it a bit more strength, so those holes locations aren't really a problem in this case.
  • Jamie Hall
    Jamie Hall Member Posts: 18,951
    Well, actually those holes in those locations are a problem. Holes in moment beams are restricted to the middle third of the beam. Anywhere else in the depth of the beam and the carrying capacity of the beam is reduced in the ratio of the square of the depth of the beam less the depth of the holes to the square of the depth of the undamaged beam. So, for example, a 1 inch hole in the top third of a 2 x 6 will, very roughly, reduce its capacity by the ratio 25/36, or even more roughly, two thirds.

    Even one hole will do this!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 4,594
    Can you draw what you mean to do?
    You want to split the loop to install another radiator and make that split at an existing one?
  • kensheets2
    kensheets2 Member Posts: 36
    edited March 2020
    just need to know if it's a problem to reduce to 1/2 out of the 3/4" sup/ret lines as shown in pic

  • Zman
    Zman Member Posts: 7,330
    edited March 2020
    Unless your radiator needs more btu's than the pipe can carry it should not be a problem. Would need more info to answer that question.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein