I would like to insert some short lengths of 3" copper into a DWV main line that is made of PVC. I will be wrapping soft copper around these short lengths and running the cold water feed for the shower through the soft copper to capture heat from the drain water.

My question is for you plumbers, what procedure would you use to insert a fitting or Tee into an existing run?

After I cut it, I can probably move the line an inch or so fairly easily but any more will be more complicated but can be accomplished.

Edit: I should add that the shower drain is too tight to the main line to put the heat exchanger in the 2" shower line.

If I wanted to add a Y into the main line and move the shower drain I would still have to cut the DWV and insert the Y. How do you guys cut in to an existing line to add a fitting? Do you use rubber couplings?

Would something like this be reliable for decades to come?


  • May I ask why heating the cold water line feeding a shower is desirable? Nov 20, 2015 at 17:59
  • I assume you're trying to capture heat from the wastewater to avoid using as much hot water at the shower? If you have cast iron DWV pipes, you're going to have issues with galvanic corrosion. If you're trying to save money by pre-heating your cold so you don't use as much hot - I doubt you'll recoup your costs within your lifetime. Unless you're getting free copper pipe. But then you'll probably make more money at the scrap yard than you'll save on your energy bill.
    – Sean
    Nov 20, 2015 at 18:08
  • @statueuphemism my wife can't get the shower hot enough so capturing some of the heat from the waste water seems worthy of consideration.
    – ArchonOSX
    Nov 20, 2015 at 20:21
  • @Sean I edited my question to add the material of my main line is PVC.
    – ArchonOSX
    Nov 20, 2015 at 20:21
  • What you linked to would work and is what is used in such a situation. I do question how much heat transfer you would get doing it in the manner that you describe. See my answer for other options.
    – pdd
    Nov 20, 2015 at 21:00

2 Answers 2


Rubber adapter couplings are what is used for what you want to do. You will need to insure that you order the correct one as copper and PVC have different outside diameters.

I do not think that you will get good heat transfer by simply wrapping soft copper around the drain line as there will be minimum contact. There are commercially available products on the market that do what you are looking for that will provide much better heat transfer. Here is one such product: Recoh-vert.

You'll notice that the pipe-in pipe method archives much greater contact between the hot drain water and the incoming cold water. They also install these on the vertical as what travels around the inside wall of the pipe when draining vertical (as opposed to just the bottom when draining horizontally). This also increases the amount of heat transfer.

You can make a similar product yourself, though I question the possibility that a home made product would prevent cross-contamination. Defiantly would not pass any local codes. Here's a video on how to make one: Shower Heat Exchanger.

If I understand your situation, you have limited space in install the heat exchanger. I believe that there are also p-trap versions available.

  • Thanks for the info. I don't know if it will be worth it but I have to consider all possibilities to save a buck.
    – ArchonOSX
    Nov 20, 2015 at 21:19

No pipe movement is needed if you use repair couplings. These are couplings with no internal stops.

The procedure is thus:

  • solder a short length of pipe into both running legs of the tee
  • measure assembly length and remove a slightly longer section of the pipe in question
  • place a repair coupling onto both cut ends of the pipe and slide them back out of the way
  • place the tee assembly into the cut area and slide the repair couplings back to cover the two joints. Ensure the couplings are centered on the cut and the tee assembly is oriented properly (I use various tools and such to prop the assembly and hold it in place)
  • solder the repair couplings into place (I use wet rags to wrap existing solder joins, to prevent them from softening)

Of course, all fittings and pipe are prepared properly (deburred/reamed, cleaned shiny, etc.) enter image description here

  • thanks I edited my question to add that my main DWV line is PVC. but something similar to that going from copper to PVC might work.
    – ArchonOSX
    Nov 20, 2015 at 20:24
  • A heavier duty version of the rubber coupler you linked would be better, something like this: missionrubber.com/images/BandSeal.jpg. The process would be similar, slide the metal band components on and back, place the rubber on the cut end, fold back the rubber boot that will seal to the new pipe/fitting, insert the new pipe/fitting then fold the rubber boot over it, slide the metal sleeve/clamps over, then tighten. Nov 20, 2015 at 20:52

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