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I have some plumbing that has been done in 3/4" PEX and the 90-degree brass elbow fittings that connect the lengths of tubing fit inside the tubing, so I don't see how these elbow fittings could possibly be 3/4" if they are inside 3/4" tubing. The elbows themselves look more like 1/2" pipes to me than 3/4" pipes.

What is the story here? Are PEX fittings smaller diameter than the tubing???

  • Ummm - yes, because physics ... It's not clear what sort of answer you're looking for. – brhans Apr 8 at 18:27
  • The inside of 3/4 pex tubbing is not 3/4. pexuniverse.com/pex-tubing-technical-specs – Alaska Man Apr 8 at 18:37
  • Because pipe is defined in trade size. The outside diameter of pipe is relevant because that's where fittings attach. So for instance early 3/4" iron pipe had a 3/4" ID and 1.05" OD by necessity. Iron pipe fittings go on the outside, so all 3/4" fittings were built for 1.05" OD and they still are. Pipe ID changes as materials do. It's like Roman chariots deciding railroad gauge deciding Space Shuttle booster size. – Harper - Reinstate Monica Apr 8 at 19:20
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The inside diameter (ID) of PEX fittings is smaller than the ID of the pipe, and the ID of the PEX pipe is smaller than the same size copper pipe. When the system was designed, the outside diameter was kept constant but the inside diameter shrank (compared to copper).

This sounds like PEX might have much lower flow rates, but the silver lining is that PEX is flexible so it has a lot fewer fittings. The change in direction in fittings of copper pipe is a flow bottle-neck even if the fittings are larger than the pipe ID. It's possible to have an entire run of PEX with no fittings at all until it comes out of the wall.

Now, even with that advantage PEX might have a lower flow rate, but it should still be much higher than any point of use calls for outside of a bathtub. Also, new PEX installs use a manifold system so you don't have 2-3 points of use coming off of one 1/2" line.

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There is three main types of PEX tubing: PEXa, PEXb, and PEXc. They differ in the manufacturing process and have their own advantages and disadvantages.

Typically PEXa is expanded over the fitting and as such maintains a larger ID through the fitting. PEXb is not expanded and uses fittings that fit inside the unexpanded tube, and as such have smaller IDs.

I can't speak to PEXc as I've never seen or used it.

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