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I'm in the process of replacing my under-sink water filtration system. The old system used 1/4 OD plastic tubing. The new system uses 3/4" OD tubing.

So I need to adapt one size to the other. Normally I would just run to the hardware store and get the right adapter, but they all just closed. So in hopes that a bored plumber is reading DIYSX tonight, I'm posting my question here.

I noted that the 3/4" tubing was labeled on the tubing itself as having 1/4" ID. So I got the bright (?) idea that I could insert the 1/4" tubing into the 3/4" tubing and get a pretty tight fight. I did so and got both sides inserted about 2" in. I pressurized the line and found no immediate leaks.

I'm curious if this could be used as a solution, or is at best a risky short-term solution. Perhaps with the right food-safe sealant, this could work?

I know I can just wait until the HW store opens tomorrow, but this info might be useful in a pinch.

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    I wouldn't even consider leaving this setup under pressure in my house.
    – Matthew
    Jan 31, 2022 at 3:17
  • Agree that it isn’t likely to end well.
    – Aloysius Defenestrate
    Jan 31, 2022 at 5:03
  • Glad I didn't leave it, then. Thanks for all the replies. It is now the next day and I got the proper push-to-connect adapter.
    – rothloup
    Jan 31, 2022 at 17:13
  • All of the answers are some variation of "don't do it, man!". I upvoted all and accepted the one that's one is most likely to be able to do when the HW store is closed.
    – rothloup
    Jan 31, 2022 at 17:18

3 Answers 3

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A simple hose clamp would take that from slightly risky to no problem. There's a very small separation force between those two tubes, and a bit of clamping would seal the deal, so to speak.

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By the way, "sealant" isn't a word used in plastic plumbing. You're either cement welding things together (usually PVC or ABS), or clamping them with gaskets and/or compression.

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    I agree with adding a hose clamp to keep it tight. Is there any concern that, without any sort of sleeve inside the inner tube, it could be tightened so much that the inner tube is squeezed shut or at least to the point of significantly reducing flow?
    – FreeMan
    Jan 31, 2022 at 17:14
  • Not really with plastic tubing. If this was rubber, sure. I wouldn't have suggested this solution in that case.
    – isherwood
    Jan 31, 2022 at 17:29
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    I've done this exact thing. I needed to use 2 home clamps oriented 180º from each other to achieve any sort of reliability.
    – noslenkwah
    Jan 31, 2022 at 18:52
  • That jubilee clip is exactly what's needed.
    – Toby Speight
    Feb 1, 2022 at 13:05
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I would have found a metal insert or sleeve to go inside the smaller pipe then slide the larger one over as you did, but then add a hose clip which would compress one pipe onto the other with the support from the inner sleeve.

Your version can fail at any time - usually due to a pressure pulse in the system.

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The usual water pressure is about 3 bar or 300 kPa or 300 kN/m2 or 0.3N/mm2.

1/4" ID pipe has 31mm2 internal surface area, which corresponds to a force of about 10N (1kg) trying to push the small pipe out for a pressure of 3 bar.

That's less force than you can apply when pushing the pipe in, so if you forced it in pretty tight, it should hold for long enough for you to stop worrying about it, then it'll come out and cause a flood at an unexpected time.

However, when summer comes, if you don't have AC, the increased temperature should make the pipes a bit softer, and you will most likely have a flood.

There is no adhesive that sticks to PE, so you really need a proper fitting.

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  • There's also the issue if something snags one of the hoses. It was simply pushed together, it could be pulled apart by accident.
    – Loren Pechtel
    Feb 1, 2022 at 3:49

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