I am repiping with PEX through out a small house (1 bath, 1 kitchen, fridge, washer/dryer).

I have an idea to running a short copper section immediately from the outside (house main shutoff) to the kitchen sink (where we would use a reverse osmosis water filter system for drinking water). After this I would transition to PEX for everything else.

Would water that touches the PEX ever run backwards into the copper line (regularly)?

Is this a thing? Do people do this?

  • WHY are you looking to make a run of copper before transitioning to PEX?
    – FreeMan
    Commented Jan 12 at 16:26
  • My main question really is just about water movement with a little bit of context ... while I do have a bit of concern about PEX (plastics) I also understand that I don't have control over the city pipes which likely use PEX and other pipe types ... whether I do the above or not, I just want to know how the water behaves in the pipes
    – farinspace
    Commented Jan 12 at 16:54
  • @FreeMan, one thought was that "new" PEX may leach somethings (VOCs) for a period of time into drinking water and I am thinking I can possibly bypass this with a direct copper line to kitchen sink before PEX transition
    – farinspace
    Commented Jan 12 at 17:05
  • 1
    Have you read/heard/seen any concern about that or is this just something you though up on your own? I'm not aware of any studies anywhere saying that PEX will leach VOCs or anything else into the water. Considering it's been in use for a couple of decades in the US and many more decades in Europe, I'm sure there'd be pretty widespread news by now if there was a worry.
    – FreeMan
    Commented Jan 12 at 17:13
  • 3
    Even if there's no reverse flow, diffusion could cause contaminants to migrate "backwards" through the plumbing when the water sits still for a period of time.
    – Greg Hill
    Commented Jan 12 at 17:41

1 Answer 1


Domestic water supply is under pressure, so under most circumstances it flows in only one direction. Normally that means it comes in through one pipe and then branches out to multiple locations, so the water that goes "out" never comes back to an earlier place in the pipes.

The one key exception is that because water expands as it heats up, there can be issues with a hot water tank causing expansion that can push backwards to the incoming water supply, which is why expansion tanks are now required in many locations.

There are also backflow prevention devices required in certain situations to make sure that dirty water does not flow backwards into supply pipes, but that is different from your copper vs. PEX issue and shouldn't be needed here.

That being said, if you are using PEX that is properly rated for potable water, it really shouldn't matter whether the water flows through copper or PEX on the way to the kitchen sink.

  • 3
    The standard example of "dirty water" is a sprinkler system where the components are often not rated for potable water so you don't want it coming back into the house.
    – Jon Custer
    Commented Jan 12 at 17:03
  • Ah, i had not thought about that .. very good example!
    – farinspace
    Commented Jan 12 at 17:08

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