According to Gary Slusser :

"George E. Cawthon" wrote

It isn't the size it is the smoothness of the pipe wall that makes a
difference in flow. Sizing pipe based on the flow needed is driven by
economics. As a homeowner, especially a do it yourself homeowner,
economics is only one factor and may not be the most important factor
for me. I would use a larger size of pipe to reduce flow noise, a
larger slower fan and larger ducts to reduce air noise, a larger size
wire to reduce voltage drop, heavier floor joists and subflooring for
greater solidity, thicker outside walls for comfort and sound control,
etc. If I eliminate the labor cost, and since labor is a major cost
in any construction, I can devote more capital toward premium
construction elements.

Problem is George you were giving him advice, not speaking as to how you'd
do it. But I agree with you, you're not a plumber. You also don't know what
you're talking about IMO when it comes to sizing water line. And if it
weren't so late, I might prove it but it is late. And I doubt you'd get much
out of it anyway. I'd really like to hear more about the smoothness and less
water noise in larger pipe though, I see a full pipe regardless of the ID or
smoothness so there will be no water movement noise. There's only one chance
to get the pipe sized correctly, and that's at the beginning when sizing it;
regardless of smoothness or whatever, it's the ID of the pipe for the job it
has to do, and that's delivering the amount of water the system needs.

George is right to a limited extent. Interior pipe smoothness _does_
have a factor in flow rates. But it's a very small one (unless the inside
of the pipe is _very_ rough), so it is not an issue in practise.

Ie: not a significant factor with (new) copper versus plastic. _Might_
become an issue with copper if your water causes the copper to crud up
really badly. Will often become an issue with old iron pipe installations.
But in either of those cases, diameter restriction is going to play a bigger
part than roughness.

Secondly, water pipe _can_ make flow noise from roughness -
microturbulance, bubble formation etc. But, if the noise is only noticable
at high water velocities - at velocities so high that pipe drag and thus
pressure drop is unacceptable - the pipe is just too damn small in the first
place - a different pipe smoothness at the same size is not going to make
a detectable difference.

Aside:

Pipe drag/pressure drop has non-linear relationship to water velocity.
In other words, pressure drop increases faster than GPMs in the same
pipe. As I recall the "rule of thumb" (at least for irrigation
systems), you should never design 1/2" lines for more than 5-7GPM, and
3/4" for more than 10-15GPM, because beyond that point pressure drop
becomes very substantial. And even those flow rates will often cause
annoying pressure problems on longer lines. Which is why the irrigation
specialist told me "for god's sake, don't use 1/2" _anywhere_" ;-).
[I cheated a bit, but only for single low flow heads.]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.