Actual PEX Inside Diameter (Size)

I was at a building supply store yesterday, and had time to spare. I grabbed a roll of 1/2" PEX and a stick of 1/2" copper pipe. I held them end to end. The OD is about the same, but the ID of the PEX is about 15% or 20% smaller, because the walls of the PEX are thicker. This means that there will be less water available at the fixture, when using PEX. For most fixtures, such as a sink or toilet this is likely not a problem, becuase those fixtures usually have a 1/4" supply tube feeding that fixture anyhow.
But for a bathtub. washing machine faucet, or an outdoor spigot, that would make the flow slightly less than with copper or steel pipe. (Probably 15 or 20% less, since the pipe is that same amount)
But it gets MUCH worse. The fittings for copper or steel pipe are always LARGER than the pipe itself. The fittings for PEX are SMALLER, since they fit inside the pipe. I grabbed a 1/2" brass PEX fitting from the shelf and looked at the ID of the hole inside this fitting. The ID of the fitting is just a tad over 1/4". Thus, about 50% of the ID of rigid copper. That is a significant difference.
While feeding a toilet or sink from 1/2" PEX may be acceptable, I'd find the amount of water at a bathtub, washing machine and particularly an outdoor spigot to be less than desirable.
I dont think any of you PEX lovers can argue with this. Facts are facts and a 1/4" hole inside of a PEX fitting is only going to allow 50% of the water flow that a 1/2" copper pipe will allow, (which is just a tad less than 1/2".)
I think this explains the reason that PEX pipe is supposed to be installed using a manifold, whereas each fixture has it's own pipe. This manifold system may be perfect for new construction, where there is a basement and easy access inside walls to install everything. But to daisy-chain pipes, from room to room, seems like the result would be completely unacceptable. And in places that have no basement, and the plumbing is being replaced in an existing building, running a huge bundle of pipes to distribute to all the fixtures in the home would be really messy.
I guess the only way to use PEX, still have adaquate water, and daisy chain the pipes would be to use 1' or larger PEX as the "feed pipe" and use 3/4" for everything else. (This would allow about the same flow as using 3/4" and 1/2" copper or steel pipe, which is common in homes).
This fact alone is what made my final decision to NOT use PEX in my home. I MUST daisy-chain my pipes, since there is no basemnt and some pipes will be exposed. Having two exposed pipes (hot andd cold) is one thing, but if I used a manifold system, I'd have 14 pipes exposed at one location. (NO THANKS)!
I did check into using 1" PEX but those fittings are even more costly and would require buying TWO crimping tools, since most of those tools are either ONE SIZE ONLY, or fit 1/2" and 3/4" combined, but not 1". Plus, I will have several outdoor spigots attached and I need adaquate flow.
Even if PEX is great pipe (as some people seem to believe), you can not deny that the fittings are TOO SMALL for some uses. Why they do not make fittings that fit over the OUTSIDE of the PEX (to eliminate the restrictions), I will never know...... I suppose there is just no way to attach them.....
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snipped-for-privacy@spamblocked.com wrote:

Do the math , Jerry . That 1/4" hole has only a quarter the area of a 1/2" hole , and thus a quarter of the amount of flow . I went with PVC ... we have a high iron content in our water here <rusty stains in the grout of our new shower> and electrolysis can be a problem .
--
Snag



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wrote:

That is right. Double or half the hole size and the flow is 4 times or 1/4 the flow. I had a house I lived in redone with the PEX. It was about 25 years old and plumbed with copper pipe. The copper kept getting pin hole leaks in it, so after repairing it several times I decided it was time to redo all the pipes. Called a plummer in and he used the PEX but I think it was the 3/4 inch size.
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On 03/10/2015 3:25 PM, Ralph Mowery wrote:

Ain't that linear, no...
First, flow is nonlinear process; there is an induced pressure drop along the run that is complex and dependent on all kinds of variables.
W/O getting into a whole treatise on fluid dynamics, one advantage of PEX is smoother surface so the length pressure loss is less--how much, precisely, I've not looked up so not sure how much an affect to assign.
But, the restrictions of the fittings are similar to a flow orifice but not as severe. There's a local pressure loss but much of that is recovered. Again, without actual handbook data and some calculations don't know a comparative flow rate after X feet of one versus the other given a set supply pressure but I'm certain it's not a factor of 4.
Somebody want to let a consulting contract? :)
--


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On Tuesday, March 10, 2015 at 4:40:17 PM UTC-4, dpb wrote:

It's already been done:
http://www.toolbase.org/PDF/CaseStudies/pex_copper_pressure.pdf
IDK if it's truly a fair comparison. They said something about previously using PEX barbed, but then switching to NPT threaded connections. Isn't that still a barb type fitting, the part that goes in the PEX? But if it's done like a typical house would be done, the result is that 1/2" PEX and 1/2" copper perform about the same.
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On 03/10/2015 4:49 PM, trader_4 wrote:

...

I'd have to read it much more thoroughly than just the quick scan but it basically confirms what I knew had to be true. As you note, digging into the details of the differences to see how much, if any, they biased it in favor of the desired result but it ain't worth the effort; net result is you'll get sufficient water either way with reasonable care in design/layout.
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On Tue, 10 Mar 2015 14:15:31 -0600, snipped-for-privacy@spamblocked.com wrote:
<snip>

You are overlooking a number of factors:
It doesn't matter if the diameter is smaller as long as it can deliver adequate flow at pressure. Max flow in 1/2 PEX will certainly be lower than 1/2 copper, but the required flow as determined by the fixture is way less than maximum flow.
Here's a link to test results comparing copper systems and several varieties of PEX systems under actual conditions:
http://www.toolbase.org/PDF/CaseStudies/pex_copper_pressure.pdf
PEX system have fewer, usually a lot fewer, fittings. The fittings in copper systems generally contribute the biggest flow restriction in any given run. In PEX, there are usually NO fittings in any given run other than the ball valve at the end.
Often the most efficient way to handle retrofits is to do a run of 3/4 to the bathroom or kitchen, and then put a mini manifold there, with individual 1/2 runs to the fixtures.
Are there situations where you have to worry about the flow rate using PEX? Absolutely. But they are far less common than you suggest, and are usually easily addressed by upsizing one or two runs or sections of runs.
I've done repiping both ways...copper and PEX, several times. After using PEX, I won't go back to copper except for old work.
Your analysis is valid as far as it goes. Yes the pipe is smaller ID. Yes the fittings are smaller ID. But in actual system configurations, as described in the above report, PEX works as well as conventional copper. There are very few situations where the difference in maximum flow rate matters.
Nobody is making you use anything you don't want, but don't try so hard to talk yourself out of trying it....
Paul F.
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On 3/10/2015 4:15 PM, snipped-for-privacy@spamblocked.com wrote:

Ever hear of someone getting the PEX ripped from the walls of their house?
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On Tue, 10 Mar 2015 14:15:31 -0600, snipped-for-privacy@spamblocked.com wrote:

One of the advertized advantages of PEX is HIGHER water flow for the same sized pipe. (due to ferwe couplings and wider bends, I imagine)

Your guess is way off.
Check out the report at www.toolbase.org/PDF/CaseStudies/pex_copper_pressure.pdf

Like I said, your assumptions are WAY off. See the report at www.toolbase.org/PDF/CaseStudies/pex_copper_pressure.pdf

Not fact.

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