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Pipe sizing for radiators

mow Member Posts: 12
I'm removing fin and tube and replacing it with cast iron radiators. There are 4 radiators totaling 30,000 BTUs at 170° 

I would be piping it with a separate supply and return.

Is 3/4 copper big enough to supply these radiators?


  • Ironman
    Ironman Member Posts: 6,372
    You need to determine the output of each radiator and then use the sum to determine the main lines.

    A 1/2" line can carry 1.5 gpm or 15k btus at std. 20* delta T.
    A 3/4" line can carry 4 gpm or 40k btus " " " " " " " " " .
    A 1" line can carry 8 gpm or 80k btus " " " " " " " " " ".
    This is at 4 fps which is the maximum recommended velocity.

    Don't mix piping methods or sizing on the same zone since water will take the path of least resistance. In other words, if you have over-sized piping (like that from an old gravity system), don't put that on the same pump as this new piping.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • tocker
    tocker Member Posts: 15
    edited May 22
    I have a similar question. Replacing fin-tubes for Runtals in bedrooms and baths that are at the far end of the house away from the boiler. To avoid running 10 ea 1/2" lines, I plan to run a pair of larger (secondary) pipe to a manifold on that end of the house, then run the 1/2" to the 5 radiators (approx 23K BTUs. How should I size the large supply/return lines between the near boiler piping and the manifold?
  • psb75
    psb75 Member Posts: 369
    The 5 half-inch ports on each 1" manifold body should be fed with 1" pipe. 5 x 1.5gpm = 7.5gpm.
    1" pipe can carry 8 gpm.
  • Canucker
    Canucker Member Posts: 685
    Unless I'm missing something, I don't see why they'd need 1 inch pipes to a manifold that is supplying a zone with a 23k btu heat loss. 3/4 inch should be more than enough
    You can have it good, fast or cheap. Pick two
  • mattmia2
    mattmia2 Member Posts: 3,168
    It depends on what the delta t is on your emitters, right?
  • tocker
    tocker Member Posts: 15
    edited May 27
    Thanks gents! You confirmed my lower math calculations at 20 delta T, I plan to run a new 1" straight run of copper to a 1" manifold. I tore out all of the old steel pipe. Two 3/4" loops out to the front and rear perimeter, then back through an old 1.25" steel down the center of the house back to the boiler room. It was full of black sludge. Thanks again!
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