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Is it OK to solvent weld PVC to CPVC? I have a Schedule 40 PVC line outside a building and need to transition to CTS CPVC for indoor plumbing (because code). I'm wondering both from a practical (will the 'dissimilar plastics' actually fuse) and legal (does UPC allow it) perspectives.

Spears and Lasco both have an IPS x CTS transition bushing (made of CPVC), I can't tell if it's only intended for going between CTS CPVC to (uncommon) Sch 40 CPVC or also perhaps for what I'm trying to do.

Of course another option is a threaded joint using MPT + FPT adapters, but since it's going to be buried I'd prefer a solvent joint.

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  • commercial-industrial-supply.com/resource-center/…. A lot of info here – Kris May 9 '18 at 19:42
  • @Kris, thanks for the link. Sounds like it's feasible. I also contacted Spears technical support, they said CPVC solvent cement (and primer if required) will join PVC and CPVC just fine. Now waiting to see if the plumbing inspector is on the same page... – Serguei May 9 '18 at 22:30
  • I would also check if the inspector would be okay if the transition were made just inside the building. If so, using an NPT connection might be more reasonable and then you wouldn't have the material compatibility concern. – UuDdLrLrSs Nov 11 '18 at 12:56
  • Yeah I ended up transitioning inside the building through a true union valve - one flange PVC other side CPVC. I avoid plastic NPT connections, only gobs of thread sealant seem to seal but then ends up in the pipes, otherwise with me the threads either leak or crack from over-tightening. – Serguei Nov 12 '18 at 18:05
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My understanding is that you can do it if you use CPVC cement - but not ordinary PVC cement.

Here is one reference (from a commercial products supply company) which confirms this:

Does CPVC to PVC Glue Exist?

...you cannot use just any PVC cement to join these two materials together. CPVC, as we explained earlier, is a stronger version of PVC pipe with a higher melting point and tougher chemical structure. This means old-fashioned PVC cement will not be fully effective on CPVC.

There is no special “CPVC to PVC glue,” so chemically joining CPVC and PVC requires solvent cement and primer that are strong enough to fully bond CPVC pipe.

To connect PVC to a CPVC fitting (or vice versa) follow these steps:

  • Brush CPVC primer on the outside of the pipe and inside of the fitting about 2 inches deep.

  • Immediately apply a light layer of CPVC cement to the area that is primed outside the pipe and inside the fitting.

  • Slide and twist the pieces together then hold them together firmly for 30 seconds.

Also, here is a reference from Weld-On, one manufacturer of (C)PVC cements which also concurs with this advice:

Q: Will CPVC solvent cement work on a PVC piping installation?

A: Yes. CPVC solvent cement technically will work on PVC piping. However, we recommend that the correct ... primer and PVC solvent cement for the particular application should be used. Please note that PVC solvent cement, because of temperature limitations, is not recommended for CPVC piping.

Now, just because it is possible doesn't mean that you should do this. For one thing it might not be to code if you used materials in a way not recommended by their manufacturer. Aside from that, in a critical system (like sewage for instance) you should think carefully if it is worth taking any risk at all.

On the other hand in a nonessential area or in some kind of temporary / emergency circumstance, this might be a reasonable course of action.

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