Old plumber wisdom, is he right or not?

[Jonathan212]

Let us not forget the plumber's claim is about pressure, not flow rate. Pressure at the point where his piping ends and devices begin. We still care about flow so we might as well ignore him, even if when he is right.

 

[Homer7]

The problem here, as in so many, many cases, is not in the pursuit of the answer but the question.
The OP said, the old plumber said ‘this size [1/2”] would give a HIGHER pressure compared to the 3/4'' option’.

Of course, there is almost no information to work with here and moreover the old plumber’s statement is an
informal logical fallacy
(wrong but not absolutely, self evidently wrong).

However, it is pretty darn close to being a formal logical fallacy and it might help to explain this to your plumber

Then again...

 

[Mary Conrads Sanburn]

[. . .]
Many plumbers have trouble with the meanings of pressure and flow. To be an old plumber now, he may well have gone into plumbing because he wasn't getting good grades... and never understood quite a lot of things, very well.
[. . .]

I had to chuckle. My husband is a Union plumber and pipe fitter. He knows his stuff. He is now retired but still fixes every thing at our home and helps his friends too.

 

[DaveC426913]

My husband is a Union plumber and pipe fitter.

How do you tell a plumber from a physicist?

Spoiler

Ask them to say "unionized".