I plan on replacing all the copper in my house with a pex system. the hot and cold pex manifolds seem much more user friendly. the copper in my house seems crazy, spliced and diced running all around. I am wondering if I can delete the expansion tank, I am thinking that pex tubing expands as it is. if not, should the tank be before the manifold? Thanks

  • Are you on a well system?
    – mike65535
    Jan 30, 2019 at 17:13
  • 1
    Can you add your location to the question? This may be a code issue. I think generally if you have a backflow prevention device, you should keep the expansion tank. Why risk damaging your pipes? You already own it, so it won't be a big deal to keep it. Jan 30, 2019 at 17:15
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    I agree with Philipp you have it keep it in the system. The more stress on plumbing the faster it will fail at some point and Murphy's law says it will fail at the worst possible time.
    – Ed Beal
    Jan 30, 2019 at 17:20

2 Answers 2


An expansion tank is only required when the water supply system has a backflow prevention device or check valve. The extra pressure caused by thermal expansion was apparently an issue in the home. I would strongly suggest that you include it in your new set up. Don't rely on the pex to expand and relieve the pressure. That could cause a pipe to rupture or a fitting to leak. Expansion tanks are typically installed in a "tee" at the cold water inlet for the water heater.


I would just leave the copper there if it is not giving you too many problems. I wouldn't say that pex is so much better than copper that making the switch is worth the time or expense even as a DIY. Steel or iron supply or drain lines are a different story but if they are functional I leave them alone too.

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