# Old plumber wisdom, is he right or not?

Why don't you check the other assumption too, that the input pressure is constant. If not, and if it drops to 50% when water flows, might the thin pipe preserve the pressure better, just by not allowing a fast flow.

anorlunda
Mentor
ping @AK_NC

You recently graduated from a college plumbing program. Are any of those oldplumber things still taught?

""Why don't you check the other assumption too, that the input pressure is constant. If not, and and if it drops to 50% when water flows, might the thin pipe preserve the pressure better, just by not allowing a fast flow. "
Because it wasn't part of the original question. Why ask now?

Normally people are concerned about what comes out of the pipe, not what's going on inside it. The flow is determined by the tap/faucet/jet/nozzle/hole on the end, nothing to do with the pipe size unless its resistance is significant compared with that of the orifice.
So what is meant by pressure? What do you mean by pressure - where? What did the old plumber think?

If you think pressure = how far a jet of water will come out of the end of a pipe with a certain sized hole in the end, it will never be further with a narrower pipe.

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.

People who use showers get very confused about pressure and flow. They usually demand pressure and don't think about flow.

We want a higher flow, that's all we want, we don't really care about pressure at any point along the way from the meter to the shower. An assumption of constant input pressure was not part of the question but was part of the answer in #17.

1/2'' pipe:

Po(1/2'') = Pi - A * f(1/2'') * L

3/4'' pipe:

Po(3/4'') = Pi - B * f(3/4'') * L

Without this assumption of the same Pi in both equations, the plumber may be right as we do care about input pressure because it affects output pressure Po and flow rate f. The plumber may be right when too many people water their gardens or have showers simultaneously, and the resulting high flow rate in the public pipe reduces the pressure where our meter connects. There is supposed to be a pressure regulator there but it cannot help if its input pressure is below the limit the regulator is set to. When this happens, it is as if there is no regulator. Then the A parameter of the public pipe comes into play. As you allow more and more water in your house, you make Pi fall in proportion to the flow. Possibly more than the pressure drop that is saved by your thicker pipe. So you end up with a lower output pressure at the balcony (Po above) and a lower flow. Thicker pipe, lower flow! The math allows for this, does anyone want to see it written down with the public pipe added? All that's missing is the parameters A, B, etc, and K for the public pipe. Anyone got those parameters?

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mfb
Mentor
A thicker pipe will always lead to more flow along that pipe and less flow along other pipes if you have multiple users. If the difference is relevant depends on various system parameters.

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.

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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...

[. . .]
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
Gold Member
My husband is a Union plumber and pipe fitter.
How do you tell a plumber from a physicist?

Ask them to say "unionized".