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Thread: Should I connect a 3" or 4" flange to 4-in drain pipe?

  1. #1
    Geotechnical Engineer Fistor's Avatar
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    Default Should I connect a 3" or 4" flange to 4-in drain pipe?

    Hi -

    The title says it all. For my toilet installation, in which I am installing a new ABS closet flange, do you recommend that I use a 3-in or 4-in flange (that is the underside diameter of the flange) to connect to the 4-in ABS riser?

    As I see it, the advantage to using a 4-in ABS is that it would connect around the outside of the riser. Disadvantage is the decreased width around the understand of the flange rim for mounting to the floor (i.e. tighter hole clearance), and perhaps that the top rim of the riser, inside the flange now, might be prone to catching or snagging material...?

    The advantage for the 3-in is that it mounts inside the 4-in riser, so no snagging, and as well, there is more width beneath the flange for floor mounting. The disadvantage (maybe) is that this is a restriction, relatviely speaking, to the rest of the drain, which is 4-in. I realize that 3-in is probably just fine for the new, low-volume flush toilets, but is a larger flange (i.e. 4-in) better?

    Let me know what you guys think, thanks!


    (extra: I am splicing this ABS riser to the old cast iron riser (need about 2-ft length) - I actually could go with a 3-in pipe spliced to the old 4-in cast iron, but I figured that keeping everything 4-in would be better - thoughts on this??)
    Last edited by Fistor; 08-23-2008 at 02:27 PM.

  2. #2
    DIY scratch-pad engineer leejosepho's Avatar
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    Default

    Quote Originally Posted by Fistor View Post
    The advantage for the 3-in is that it mounts inside the 4-in riser, so no snagging, and as well, there is more width beneath the flange for floor mounting. The disadvantage (maybe) is that this is a restriction, relatviely speaking, to the rest of the drain ...
    Yes, but the discharge hole in the bottom of the toilet is only around 2", thereby restricting the actual flush going into a pipe about twice its size while joined at a 3" opening. Personally, I use 3" flanges and 3" pipe inside my house since the opening at the bottom of the toilet is less than the pipe i.d. and nothing is very likely to catch on the top edge of the riser as long as it has been de-burred.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    If you have a 4" coming up, it would seem almost easier to just couple on some more 4" and use the 4" hub flange.
    You do need to cut a round hole if you go that way.

  4. #4

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    I prefer the 3" flange because of it's larger surface, too. The problem, as I see it, is that you have a 4" pipe in the first place. I can think of no reason for a residential closet pipe to be more than 3".
    Steve's Plumbing Service

  5. #5
    One who lurks Basement_Lurker's Avatar
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    Having a 4" pipe there is a luxury...take the easy way out and use a 3" internal flange..using a 4" flange will make no difference in terms of functionality of the "shit-hole"!
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


    www.blackbirdkitchenandbath.com

  6. #6
    Master Plumber Redwood's Avatar
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    I like using a 4" flange the first time. It leaves other options open for flange replacement in the future.

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