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Thread: Problem flowing 4 inch PVC into 3 inch cast iron?

  1. #1

    Default Problem flowing 4 inch PVC into 3 inch cast iron?

    Looking to see if I've made a mistake here. I've got an old (1920s) house that I added about 30 feet of 4 inch PVC pipe to drain two bathrooms into an existing cast iron drain that goes out to the street. The existing cast iron pipe outer diameter measures 3.3 inches (circumference= 10-3/8). Is this a "3 inch" cast iron drain pipe size?

    Is it OK to be flowing from the larger 4 ID to a smaller 3 ID or am I asking for blockages here? I made the connection using a rubber sleeve (with hose clamps) that fit on the cast iron hub OD and the OD of the PVC, so the flow path gets smaller by going through the cast iron hub.

    Thanks for your help.

  2. #2
    Master Plumber master plumber mark's Avatar
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    Feb 2005
    indianapolis indiana - land of the free, home of the brave....
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    Talking its ok

    it would have been better if it were 4 inch all the way ,
    but you got to do what you got to do,,, thats your
    mistake you got to live with.

    I doubt the fixture units you have added
    will make much difference going into the 3 inch

    my only advice might be to put some sort of clean out
    at where you have to reduce from the 4 down to the
    existing 3 inch cast pipe...

    makeing the reduction on a verticle drop into the
    3 inch pipe might be wiser . ---instead of on a horizontal
    Also reduce down with a 3x4 pvc bell reducer for better flow.

    it will most likely work fine as long as you have a
    cleanout at the bell near by just in case it were ever to stop
    up on occasion

    wait and see what happens
    Last edited by master plumber mark; 08-22-2005 at 03:50 PM.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona

    Default reducer

    You have created a lifetime of aggravation for yourself. You NEVER reduce the size of a drain line in the direction of flow. If you could find an eccentric reducer so that the bottom surfaces were level, then it would be less of a problem, but the way you have done it, there will always be a "pond" of water at the reduction. That pond is going to cause the sewage to slow down, and the solids may stay in the "pond" while the water continues on down the drain. Your problems will be the same as if you had a sag in the line.

  4. #4

    Default sketch added

    Thanks to both repliers. I thought I'd add a sketch to help show the connection I made that I'm concerned about. I connected the PVC to a cast iron WYE that is horizontal (pointing slightly down pitch). While I won't have the puddling suggested, I also don't have the benefit of going into a vertical run. This is in a crawl space at ground level as the cast iron is going to the sewer. I did put a 3 inch clean out about 30 feet upstream in the 4 inch PVC. Its in a closet on the main floor. Any more thoughts?
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  5. #5
    Master Plumber master plumber mark's Avatar
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    Talking oh , it will probably work....

    well it will probably work out ok because you
    are on a verticle drop into the wye.... you got lucky there..

    its all done now, but a clean out near that spot might be
    good...but if you put one back about 30 feet , it just
    means you got to get a cable with at least 40 foot on it...

    by the time the water gets down the line 20 feet or so
    it is going to be running pretty flatin that 4 inch pipe
    and will most likely not know the difference when it gets
    dropped into the 3 inch...

    if you used a 4x3 FERNCO it should slide right through with
    very little aggravation

    Of course, this isnt anything I would be particulary proud of,

    mistakes do happen, but I would not be bragging
    too much about this one

    and you might want to hide that mess under a mound of
    dirt if you ever plan on sellling the
    house someday...or it could very well come back to haunt you..

    but all in all it will most likely work for a long time...

    just wait and see
    Last edited by master plumber mark; 08-22-2005 at 06:03 PM.

  6. #6
    DIY Senior Member finnegan's Avatar
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    Aug 2005


    I agree. While this is wrong, it should work ok. I would not bother with adding the clean out until you need it.

  7. #7
    General Contractor Carpenter toolaholic's Avatar
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    Jul 2005
    Marin Co. Ca.

    Default hope an inspector isn,t going to see this one!

    if so ,you,re in trouble good luck

  8. #8
    Tradesman Plumber Kristi's Avatar
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    Jan 2005
    Vancouver, BC


    sorry, last minute question when you're already well and finished with this one... two bathrooms? why didn't you make it 3" instead of 4"?

  9. #9

    Default why not 3"?

    Oddly enough the house has two different sewer connections. One running to the north that serviced the bathrooms, which was 4" cast iron,and this one I tied into which is the 3" going to the south, servicing the kitchen/laundry. The 4" had tree root problem that would clog up once/year and so I wanted to stop that. Then I was worried I traded one problem for another. I actually didn't realize the one to the south was only 3" until I ran the new 4" PVC over to it (about 30 feet) and then went to tie in. Its in a separate crawl space for the kitchen addition versus the main house.
    Last edited by skip; 08-26-2005 at 11:54 AM. Reason: clarify


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