This took me by surprise, but I was reading the Residential Installation Instructions for Uponor PEX A, and it says,
Do not install Uponor AquaPEX pipe between the tub/shower valve and tub spout.
What's the reason for this restriction? Why would this stretch of pipe be different then any other pipe between a valve and a spout?
Why can't I just get get an NPT adapter and plug on a spout?
The warning is near the beginning of the manual. Later, when discussing shower and tub connections on page 28, the manual says:
Caution: Do not use Uponor AquaPEX piping to connect the tub and shower valve to the tub downspout as this may create excessive back pressure in the valve, causing it to remain slightly open.
Emphasis added. That's your answer.
The warning is near the beginning of the manual. Later, when discussing shower and tub connections on page 28, the manual says:
Caution: Do not use Uponor AquaPEX piping to connect the tub and shower valve to the tub downspout as this may create excessive back pressure in the valve, causing it to remain slightly open.
if in fact that statement is made by Uponor, it is in contradiction to a simple google search of pex outflow copper which returns this as one example
Does PEX have the same flow as copper? Due to the minor differences in the tub/shower valves, the flow rate at the Test Fixture (TF) was slightly different between the PEX and copper systems. The PEX TF > hot flow rate was measured at 1.7 gpm, while the copper TF hot flow rate was 1.5 gpm https://www.homeinnovation.com/-/media/Files/Reports/pex_copper_pressure.pdf page 7 of 11
I have always known pex to outflow copper... 3/8 pex outflows 1/2 copper.
The statement of pex creating back pressure between valve and open spout is idiotic.
I would not use a straight length of pex, which would be about 4-5 feet or less, between the valve and tub spouts because of thermal expansion. Something like 1" per 100' per 10°, that's why you run expansion loops for runs of pex. Having pex from shower valve to spouts, behind the tub/bath walls not easily accessible, if expansion forces from 50°F to 120°F temperatures everyday eventually pop it off will create a major headache, worse than the pvc cloud in ohio currently. For this one scenario using copper done properly between valve and upper & lower spouts would be better.
I wrote to Uponor and received an answer. In the common installation where the shower valve tees to a spout and shower head, and the diverter is in the spout, the path to the shower head is always open. A little bit of back pressure from the spout side of the tee can cause some flow out the shower head. The warning is about that situation.
Their email also advised to consult with the shower valve manufacturer. That was in response to the suggestion that if you have a separate diverter valve and no open path to the shower there seems to be no reason to avoid PEX to the spout.
It all seems somewhat ass-covering, and in my own evaluation probably in response to situations where PEX was used to install overcomplicated or just poor plumbing between valve and bath spout. Obviously in most situations where the spout is a few inches below the valve a simple prefab metal pipe is called for. The other answer to this question from @ron adds an interesting twist to this whole story.
To answer the question, why can't I just use an NPT adapter and plug on a spout?. If you have a spout that screws into a drop ear you almost certainly don't have the diverter in that spout and you can connect the valve to the ear with any suitable pipe. That's my assessment. Similarly, if you have a true diverter valve or one that is integrated into a multi-Way shower valve, you should follow the valve manufacturers instructions and you can ignore this warning from uponor.
If you wanted to bother, you could get (well, I'm not sure Uponor specifically, but PEX fittings generally, they are available) a 3/4 PEX to 1/2 MPT for the valve end, and a 3/4PEX to 1/2" FPT drop-ear elbow for the spout end. You generally still need a short chunk of threaded pipe or copper tube (which depends on the spout) for the actual spout connection.
Simpler to just do that part in copper tubing, though. Quite possibly also cheaper.
Don't know that I buy the back pressure argument from uponor, but from a practical sense, trying to tie pex from a valve directly to tub spout isn't likely to work. The angles involved don't lend themselves to bending pex around them, and in almost every situation except roman tub fillers, the spout itself is in some way supported buy the plumbing it connects to. Trying to do this in pex is likely to lead to a tub spout loosely connected to the wall and/or sagging under it's own weight.
Whether the back pressure argument holds up (hard to imagine the solenoids in my sprinkler setup or the washing machine are slamming shut slower than a human pulls up a diverter knob), it's generally a better idea to run a stub-el to the tub spout location. And like jay613 comments above, you might only have a few inches between the bottom of the valve and the spout location, which probably lends itself better to using Cu for the whole run.
The answer is because Uponor sells a proprietary "tub spout" called ProPEX lead-free (LF) copper tub ells (L). You're supposed to go AquaPEX to that tub ell. And they make many different types of them too including 1/2, 3/4, 1 with different lengths. From that ell, you can affix the tub spout using a compression fitting.
