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Thread: Connecting PVC section to damaged Vertical Cast Iron of Different Diameter

  1. #1
    DIY Junior Member will2sail's Avatar
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    Default Connecting PVC section to damaged Vertical Cast Iron of Different Diameter

    I had an 80 year old section of cast iron split down the side. I cut out the section and planned to simply install a section of 4" PVC with no-hub couplings. I read that the proper couplings were those wrapped with metal. I tried to do this, but the cast iron is 4.25" and the PVC is 4.5". When I tightened the clamps the metal buckled and the clamp slid sideways due to the difference in diameter. It did not appear I would get a good seal. I came up with the following solution and would welcome feedback.

    I could have used longer rubber connectors and they would accommodate the different diameters, but I didn't want water sitting inside the lower coupling where the top of the cast iron was cut. This seemed like a leak waiting to happen and a place for any material to get caught and clog the pipe. I bought a section of 3" PVC and two 4" to 3" rubber adapter couplings and combined these with a short section of the 4" pipe and a 4" to 4" long rubber coupling. At the top I simply used the reducing coupling to connect the cast iron to 3" PVC, beveling the inner edge of the PVC and lowering it enough that the top is inside the smaller diameter of the coupling so there won't be a trap collecting water. At the bottom I left the 3" PVC long enough that it extends down inside the cast iron pipe an inch or more which ensures no water collecting here unless the pipe backs up. To seal the bottom I slipped a 4" to 3" coupling up the 3" pipe followed by a short section of 4" PVC pipe and one of the long rubber connectors. The bottom of the 4" PVC is sealed to the cast iron with the long coupling and the top to the 3" pipe with the 4" to 3" adapter. The bottom is held snugly in place by being inserted inside the cast iron pipe. The top is only supported by the rubber adapter. There should be no side load so this may be adequate but I am wondering if I should support the top of the pipe below the coupling with a strap or by foaming it into place.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you had gone to a real plumbing supply store, you could have picked up the proper coupler and not ended up with some kludge that, while it may work, doesn't pass code.

    It can be quite dangerous cutting a section of CI pipe out of a run. Depending on how things are assembled above it, that section which could weigh hundreds of pounds, is now hanging there. The PVC is NOT designed to hold that weight. If you don't have any, you really should pick up and add in some proper clamps to hold that section so it doesn't gradually creep down as it slowly deforms the lead in the joints above.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Forum Admin, Expert Plumber Terry's Avatar
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    If it's old cast iron, it may have an OD of 4-1/8"
    If that is the case, I use a copper by cast iron no-hub mission coupling. A standard No-Hub coupling will twist.
    Copper x no hub cast iron

    Pipes can move quite a bit sidewise given some time. I would get the correct couplings.

    Last edited by Terry; 09-07-2013 at 08:57 PM.

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    Moderator & Master Plumber hj's Avatar
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    quote; Depending on how things are assembled above it, that section which could weigh hundreds of pounds, is now hanging there

    The ONLY section that could be "hanging there" and possibly fall down would be the section going through the roof. EVERY other piece has fittings on it which are held by structural members keeping them in place.
    Licensed residential and commercial plumber

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    DIY Junior Member will2sail's Avatar
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    Thanks for your comments. I did purchase a riser clamp and support the top section of cast iron (~3 feet) before cutting out the damaged area so that isn't an issue.

    I believe I would have had to order the mission fittings and my wife was not in a waiting mood with laundry piling up. I have time to change this if I can get the proper fitting, but I am not clear on what to buy. I have 4.25" OD cast iron and would then go to 4" (4.5" OD) PVC. Proflex lists the 3000-44 for 4" CI to plastic, but Terry mentions using the coupling for copper so should I use the 3001-44 for CI to copper?

    Jim, I did go to two proper plumbing supply houses looking for fittings that would work. I am not a pro either, a retired nuclear engineer. I am not against using a plumber, but my favorite works by himself and his wife is critically ill at the moment.

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    DIY Junior Member will2sail's Avatar
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    I contacted Fernco for recommendations and the two couplings they recommended are the unshielded ones that I am using.

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    Forum Admin, Expert Plumber Terry's Avatar
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    They do sell them. To me, that explains his reasoning.


    I wouldn't be able to pass inspection with them though. I'm not surprised. That's why we have plumbing inspecters, permits, apprenticships, testing and continuing education.
    It' 6,000 hours working with a journeyman for the residential license.
    They expect more of plumbers than that.

    I do this for a living. I would never use the coupling you installed.

    I'm redoing a previous master bath remodel in a classy part of town. On the same street as the Gates Mansion.
    They had run a soaking tub and a shower down the 1.5" kitchen vent on the floor below. No inpection on that job either.
    I now have to find a place for the 2.0" pipe. It looks like I can drop down and through the wall between the kitchen and dining and into the crawl space where the other plumbing is.

    No hub is 4-3/8" and copper is 4-1/8"
    I would have gone with the copper by no-hub. It works.
    You can also fine a no-hub x plastic, but unless it's snug to the old cast, you're better off with the copper sizing.

    Last edited by Terry; 09-17-2013 at 03:34 PM.

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    DIY Junior Member will2sail's Avatar
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    Thanks Terry. You are a gift to us unwashed trying to figure these things out. It wasn't easy, but I found a Mission coupling that I think will do the trick.

  9. #9
    DIY Junior Member will2sail's Avatar
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    One thing that confused me, not being a plumbing professional, was the "mission" coupling. It was not capitalized and I did not realize it was a brand. Initially I thought it was a type of coupling.

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