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What are the proper ways to connect PVC to galvanized metal?

The context of this question is for a house that has a kitchen drain stack made of 2" PVC, except for the topmost (vent) section, which is 1.5" galvanized metal. The metal pipe is connected to the PVC stack by one of those flexible couplings made by Fernco.

If there is too much water in the sink draining at once, water will actually go up the vent portion and leak out of the coupling at the top. It leaks with just enough force to squirt water up about a half inch.

Plumbing diagram

PVC stack transitioning to galvanized pipe

Coupling removed:

Coupling removed

The metal pipe is of course corroding on the inside, but not bad enough to cause major blockage or anything.

Photo of metal vent pipe from below, looking up:
(Sorry, it was hard to get my phone to focus just right) Galvanized pipe stack inside

Is there a better way to for this to be connected? I don't know how to transition from PVC to galvanized metal.

I tried tightening the clamps on the coupling, but it didn't help. I'm worried that tightening it too much could deform the pipe and make things worse. I also tried seating the coupling slightly higher up... still no better.

I found a video of someone using a thread sealant between a galvanized pipe and one of these flexible couplings... could that really help? Or perhaps make things worse? They used Blue Monster sealant:
https://www.youtube.com/watch?v=G1FMy10oqao

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  • That appears to be a plumbing vent. Perhaps you can replace the entire galvanized pipe with PVC. If not, perhaps you can replace enough of the galvanized pipe with PVC so the coupler is up much higher....away from where the water might reach when backing up.
    – Steve Wellens
    Jun 6, 2021 at 4:40
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    It seems to me the real problem is the water backing up in the first place. Maybe your drain needs snaked/augered out to the street. The bigger pipe should be able to accommodate the discharge from the smaller pipe unless there is a blockage downstream.
    – UnhandledExcepSean
    Jun 6, 2021 at 5:07
  • Those rubber couplings are particularly easy to get a good seal with. you may wish to sand the outside of the metal pipe if the surface is too rough and or squeeze a layer of plumbers putty over any significant dents, but is your coupling sized correctly? it's better if you have to stretch it slightly to get it on than to compress it to a smaller size.
    – K H
    Jun 6, 2021 at 5:08
  • Second ExepSean's comment as well. Based on the heights in your diagram, a clear drain would not allow this far back up.
    – K H
    Jun 6, 2021 at 5:09

2 Answers 2

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First off,seconding what others have said. It shouldn't back up. Fix that and find out why.

Second, its often a good idea to do "belt and braces" (over careful) when doing infrastructure stuff that you don't plan to even move again and will "just work".

At the moment your proposed pipe just has one leak prevention method - the grip of your coupling on the plastic pipe below. But plastic distorts under pressure and (very slightly)heat, and your coupling isn't gripping it hard enough to close all the gaps, or to keep them closed if the pipe isnt free flowing.

There are 2 ways to address that.

First, have the top pipe actually going * inside * the bottom pipe, and then the coupling. That way water should be directed down and not press against the coupling seal in the first place.

Second - and i want to emphasise this should not be needed, if the bottom pipe is cleared - the gap between 2 and 1.5 inch is quite a bit. Knock up some kind of ad-hoc plastic shim material between them from a pipe offcut, to take up the space between them, or an appropriate reducer if one exists. Use PVC cement to bond it to the inside of the PVC pipe, so the metal pipe is a tight fit (with plumbers lubricant if needed). When that's done, the shim won't be perfect. But if you clean and sand down the area, you can then use epoxy adhesive to bond and fill the spaces that the shim didn't seal. That should leave you a snug fitting hole and the metal pipe inside it. More important it should mean the plastic pipe is more rigid against the coupling. It shouldn't leak if the bottom pipe is clear. And * then * put the coupling over the entire area to block leaks.

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  • Sorry but I don't think I was clear: The metal pipe is just the vent. (Take a look at the diagram, there's nothing else in that particular stack). I don't think that having the metal and PVC overlap alone would help anything, since the water is going up the vent.
    – Bort
    Jun 6, 2021 at 13:02
  • I'll look into a shim solution. The OD of the metal pipe is 1.9", and the ID of the PVC is 2", so IDK what solid material I could fill the gap with. Maybe just expoxy alone.
    – Bort
    Jun 6, 2021 at 13:07
  • Your main problem is that the water is backing up. The secondary issue is that the connection isn't good so it leaks out at the join, rather than * just * backing up. You have to address the first of those. A shim and insert is more, extra safety on top, to stiffen/brace the PVC pipe better against the metal pipe, so the coupling can really tighten properly against it, because the join on a waste pipe should be watertight and gas tight (gas from waste and sewers) even if it doesn't ever leak * water * visibly again.
    – Stilez
    Jun 6, 2021 at 17:28
  • I understand the backup issue. What I don't yet understand is this part: "That way water should be directed down and not press against the coupling seal in the first place."
    – Bort
    Jun 6, 2021 at 21:39
  • Suppose for whatever reason, the water level is above the coupling, and the PVC pipe is blocked, or not allowing it out fast enough to keep the level below the coupling. At that moment, the water in the steel pipe and the area within the coupling, will try to reach a lower level by any means. The water is directly in contact with the coupling-to-PVC joint. If the coupling has even a small gap due to PVC pipe distortion, it'll drip. If there's pressure from the buildup it'll tend to push the gap between coupling and PVC if it can. But if the steel pipe is * within * the lower pipe, .....
    – Stilez
    Jun 7, 2021 at 0:31
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First - a leak here is bad regardless of water leakage or not. If it leaks water sometimes, it leaks sewer gas all the time.

You could try just a new rubber coupling. Perhaps the one you have is somewhat damaged. Examine the pipes carefully and remove any sharp edges that might damage the new coupling before attempting to install it.

You could also cut the galvanized higher, and reduce to 1-1/2" in PVC, then make the join with a straight, "shielded" (metal armor over the whole coupling, under the clamps) rubber coupling. I guess they do also make (I've never seen one in the wild, so likely have to order it) a shielded 1.5 x 2 inch with just thicker rubber on the 1.5 inch side.

In some fantasy world you could do it with pipe threads, but the hassle of getting that pipe threaded in place is prohibitive in the real world.

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