Replace lower section of cast iron plumbing stack with PVC: good idea, or not?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by temp945, Jul 30, 2020.

  1. temp945

    temp945 New Member

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    Florida
    I'm currently doing a bathroom remodel on a 1960s home. The horizontal cast iron drains in the crawlspace were replaced a while back with PVC. The plumbing stack transitions from PVC in the crawlspace to cast iron which exits the roof.

    Just above where the plumbing stack transitions from PVC in the crawlspace to cast iron, there are two horizontal fittings for two sink drains. I have attached a couple of photos below.

    IMG_2305_(Medium).jpeg

    IMG_2307_(Medium).jpeg

    I was originally planning on sweating copper slip joint fittings to the ends of the 1.25" copper drain pipes in order to connect the new sinks. But a local plumber that I've talked to says that the better plan - since the walls are open - is to replace the 3-foot lower section of the cast iron stack and the sink drain fittings with PVC.

    What are your opinions on the plumber's plan? It sounds a bit more complicated and laborious to me, but it also sounds like a good idea to get rid of the lower section of cast iron drain pipe.

    Thanks for reading!
     
  2. Jeff H Young

    Jeff H Young In the Trades

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    I don't like your connection PVC to castiron . So that tips the scales in my opinion to get rid of the cast from under house to above flood level of sinks. be very careful that 4 inch stick in heavy use a CP44 or equivalent coupling and not unshielded rubber to connect cast to PVC
     
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Hopefully, that vertical section of CI is properly supported, otherwise, all of that weight will end up warping the PVC, and potentially, cracking it. They make special hangers for the CI on the vertical to hold the weight to prevent that.

    One thing to consider is that drainage through CI will be much quieter than through PVC. So, unless the CI is worn out, it can be nice to leave it. But, if it's only serving as a vent, drainage noise is a non-issue. A second thing on PVC is that it does expand and contract LOTS more than CI, so you need to account for potential thermal expansion and contraction when draining hot water from say a shower into it...it can squeak, pop, or otherwise make noises if it's tight anywhere as it moves, or (shouldn't be an issue in a typical home because of the short lengths), bow between supports when it expands, or try to pull apart when it contracts depending on when it was installed at what temperature.
     
  5. Jeff H Young

    Jeff H Young In the Trades

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    Good point from Jadnashua make sure stack is supported!
    I would not consider noise at all in deciding whether the stack should be cast or PVC in this case . that 18 inches between the floor and the santee is negligible. If that is going to sway you then my suggestion would be pipe the entire system in cast . I wouldn't do it on my house but If I had a customer wanted all cast I would gladly do it and tell them the same. but some feel the quality is worth it ,to which I don't argue. you could also repipe that small section with cast iron but again to me not worth it but to someone that wants a rolls royce it is
     
  6. temp945

    temp945 New Member

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    I'm now planning on removing all the cast iron vent and sanitary tee and replacing it with PVC. In that case, I think I'm going to try the job myself since I am confident in working with PVC pipe (but not copper and/or cast iron).

    I have access to a "soil pipe cutter" to help me break down the cast iron pipe. I will start in the attic and remove it in small, manageable pieces, working down to the crawlspace. After I remove it all, my plan is to just replace it with 4" PVC and a 4" x 1.5" x 1.5" sanitary tee:

    https://www.supplyhouse.com/Spears-P429-419-4-x-1-1-2-PVC-DWV-Double-Sanitary-Tee

    Do you see any problem with my new plan of action?
     
  7. Jeff H Young

    Jeff H Young In the Trades

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    just be careful sometimes i just bash it with a hammer chards can fly the soil pipe cutter a bit safer.
     
  8. Terry

    Terry Administrator Staff Member

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    [​IMG]
     
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  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Be careful with longer runs of heavier gauge pvc...while the cement may make it hard to disconnect the pieces after maybe 10's of seconds, it can creep when under pressure, tension, or torque for much longer if not well anchored or supported. The cement literally melts the plastic, and once the solvent all evaporates, it is solid, but prior to that, things will stay soft and things can flow a bit. Especially with larger diameters, it can tend to push the pipe out of the hub some since the hub is tapered.
     
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  10. Jeff H Young

    Jeff H Young In the Trades

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    you bet it does turn your back and it works its way out even worse its still partialy in and it wont budge cant pull it apart. watch out for that as well
     
  11. temp945

    temp945 New Member

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    Thanks Jim and Jeff - so when gluing the larger 4" sections, I should make sure to push and hold the connections while they cure? How long do you suggest keeping pressure on the joint - 60 seconds long enough?
     
  12. Helper Dave

    Helper Dave Member

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    30 seconds is usually plenty. And key to getting 4" to sit all the way into a fitting is to give it a good 1/4 twist as you put it in. Not possible when you're putting fittings up against a wall, but it'll work on the straight pipe sections.
     
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  13. Jeff H Young

    Jeff H Young In the Trades

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    Hadnt done any pvc in a while wednsday I had a small job partly under concrete. I bought brand new glue and primer (mine looked good but was old) I was super care ful as water presure is around 120 held every thing tight forever, ha ha 100 degrees out and waited 6 hours befor putting any water on. I was a little paranoid as the customers house was shut down . all turned out good but I go out of my way to stack odds of success in my favor. Helper dave is right about half a minute and a twist is good .
     
  14. temp945

    temp945 New Member

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    Thanks Dave and Jeff!

    Another question: the existing cast iron vent exits the roof and is covered with a lead boot. Does anyone know if 4" PVC will fit inside of a 4" lead boot? I don't know if the lead boot is old enough that it was made to specifically fit cast iron. If possible, I would just slip the new PVC vent up into the existing boot without removing the boot from the roof. However, the O.D. of the 4" PVC is a bit larger - is it too large to fit?

    I can always keep the portion of the cast iron that fits into the boot, and then connect it with a shielded coupling in the attic to the PVC vent below. The next time the roof shingles are redone, the old lead boot and cast iron pipe can be replaced with a short section of PVC and a new boot.
     
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  15. Jeff H Young

    Jeff H Young In the Trades

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    I prefer not to mess with roof penetrations , for one reason Im probebly not going to get paid more, dont want to go on roof , dont want to have to be responceable for the roof leaking. Thats partly why I said just cut it at 42 above floor there would not be a single joint left to leak . but running it at least into attic is better . I might stop there . but if you hurry and gettr done thru the roof Isias is going to give you a roof test real soon.
    Hang tight batten down!
     
  16. temp945

    temp945 New Member

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    I agree - I don't want to mess with the roof penetration if at all possible! But it does look like I might be able to just slip the new PVC into the old boot without removing the boot or doing any work on the roof at all.

    Maybe my plan to run the new PVC into the attic and just use a coupling to attach to the roof penetration is best.

    I sure hope Isias doesn't cause too much havoc. A major hurricane is the last thing anyone needs right now!
     
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