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Should we up-size when converting from 3/4" copper to pex around water tank?

I am replacing a leaky valve on a section of 3/4" copper pipe on the feed-side of my water heating tank, with PEX-B (with crimp rings). And wondering what PEX size to go for.

I purchased 3/4" pex and fittings because some sections on the output side were already on 3/4“ PEX (they had been replaced by the previous owner).

But I am reminded once again how much of a difference in the inner diameter there is between PEX fittings and copper fittings of the same denomination… and wondering if maybe upsizing would have been more judicious choice.

This image shows a piece of copper 3/4" tubing i cut (not a crimp ring) on a 3/4" pex-b brass fitting.

copper ring on brass pex fitting

I don’t currently experience issues with hot water pressure in my (2bath) condo, but I am wondering if some might develop as more sections of copper get swapped with 3/4" pex brass fittings along the line.

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While PEX is smaller, it's also smoother and if you use it right there are less sharp corners that have a high friction cost (use PEX as PEX, that can bend smoothly in large radius corners, rather than using PEX as if it was rigid pipe that needs a fitting every time you change direction...which far too many plumbers do, presumably because they think that way, or they get to sell more expensive fittings that way...)

Likewise, even if you don't adopt a pre-built manifold, using a manifold-like layout on your system (where more inexpensive tubes run to each fixture from a point near the supply, rather than everything sharing one pipe until it branches off near each fixture) will limit pressure losses under flow conditions.

However, in my area, there is a code modification (amendment to an adopted model code) that wants all piping (cold or hot) within 3 "as the pipe goes" feet of a water heater to be metallic. If you're not in Vermont that particular one may not apply to you, but do check your applicable code for any such "gotchas" before proceeding.

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You should not notice a great difference in pressure. Flow will be reduced by a minor amount. Most all of the fixtures we have in our homes already have a reduction at the output down to 3/8 or 1/4 inch.

If your water pressure is X it will be X throughout the system until any pressure reducing device is encountered.
Total flow will be less with the Pex. You may see it takes a slightly longer time to fill the bathtub.

In real world I have changed copper, CPVC, and iron plumbing to PEX without any noticeable change. ( other than eliminating the leaks that existed.)

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