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Boiler flue sizing??

area of a flue pipe:

A = 3.1416 X radius squared

or A = .7854 X diameter squared

Circumference of a circle is:

C = 3.1416 X diameter

An example is 4" flue area is 12.56" (A = 3.1416 X 2 (radius of a 4" circle) squared)


  • al leone
    al leone Member Posts: 25

    What was that old smoke pipe sizing formula from school?
    Was it D2+1/2d or something like that. Does anyone remember how it goes.
  • FRANK_24
    FRANK_24 Member Posts: 80

    d squared x 5 = maximum btu for that size pipe
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,557

    a 6" flue would be:

    6 squared or 36 X 5 = 180 BTU's ???
  • bob young
    bob young Member Posts: 2,177

  • The area of the flue

    in inches determines btu as per the American Gas Association rules of 4500 to 6500 BTU's per square inch. It is then determined by the manufacturer which btu per square inch they will use.

    On newer high efficiency equipment those rules do not pertain.
This discussion has been closed.