PVC hub to cast iron hub connection

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Old 08-30-16, 12:52 PM
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PVC hub to cast iron hub connection

I'm working on a bathroom remodel and needed to alter the plumbing some so I cutout the existing 4" cast iron back to the nearest vertical pipe section hub. Now I need to convert to pvc at that cast iron hub and typically you would use a fernco type 'donut'. I found that due to my space restrictions, I need to make a 90 then get into a wye as close as possible. The only way I find this works well is to use a street 90, but with the hub end of the street going down into the cast iron hub. I've never made a connection like this but the pvc hub actually fits very nicely into the cast iron hub. Of course this isn't standard and there's no off the shelf donut that would work, so I'd have to seal it with caulk and/or glue.

I really don't see any problem with this connection (it'll probably be more robust than a donut connection) but wanted to ask if anyone else does. Also what type of sealant would you use?

Included photo shows the street 90 into the CI hub, 4" donut also shown for reference.
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Old 08-30-16, 03:13 PM
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I don't know of any connector for this. I would not attempt any type of glue or caulk.

It's tight but it looks like there's room for a no-hub connector. You need to get an angle grinder in the wall and cut off the cast iron hub. You can then transition into PVC pipe.
 
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Old 08-30-16, 05:25 PM
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Whoa, seriously? You would sacrifice an in-hub joint so that you could make a butt-joint and wrap it with rubber and clamps?? I've certainly used those fernco connectors before when needed but I've seen how easily the joint can leak if there's not perfect pipe alignment, rough surface on the cast iron, or any small shift in the house/plumbing later. The in-hub joint, even with a 1/8" air gap all the way around, is already nearly leak proof from everything but a full blown backup.

Why wouldn't you want to glue or caulk it?
 
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Old 08-30-16, 06:20 PM
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The banded no-hubs should not be confused with a flexible Fernco coupler. The flexible connectors fit loosely on the pipe and should not be used.

The no-hubs are stainless steel, have a center stop, and IMO are as good as a glued joint or better. They are the standard for repairing pipe.

Cut and clean the cast iron, the surface doesn't need to be perfect but cut as square as possible. Tighten the screws to 60 inch pounds.

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Old 08-30-16, 08:57 PM
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As I said, I've used those fittings before (the PVC to CI steel banded no-hub couplings just like the one you pictured). I've used them myself and dealt with them many times already installed. I'll use them again, but given the opportunity to have an in-hub coupling like I can, I'll take that over a fancy strap any day.

I've never seen a pvc glued joint spring a leak. Have seen banded no-hub couplings leak many times. In my area all the houses are 100+ years old. I can't tell you the number of no-hub couplings I've seen with rust and water marks down the pipe because it leaked.

Anyone else have thoughts about what type of sealant would work well for my application?
 
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Old 09-01-16, 02:01 PM
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Caulk or silicon might work, but it's not a code-compliant nor typical industry solution. While it may work, you probably won't find a plumber here to recommend it. It may work, it may fail... we just don't know, so most won't recommend it.

I'm wondering if you could flip the street elbow around and use a donut, have the elbow come out along the wall, then use another bend to get into the wye a bit further "down" (down in your picture).

Just a thought...
 
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