I am curious about the connection from a tub/shower valve to a tub spout, where manufacturers warned against using PEX tubing due to the smaller ID of PEX and the even smaller fitting if it's PEX-B. The restriction causes a back pressure that results in water coming out of the shower at the same time.

I don't have this situation just trying to understand that better.

If I have a typical mixing valve with a cold and hot feed on the left and right and a top port to shower and a bottom port to the spout, how can water be backed into the shower line? Unless the top port to the shower is open? That can't be right?

Or is this restriction only an issue if you use a twin-el where water is sent to the tub spout and the line to the shower is always open from the twin-el?

If that is the case, the line from the twin-el to the shower head is ok to be PEX? The restriction of PEX is ok? We know many shower heads have built in restrictors, if flow out of the shower is reduced significantly, and since a twin-el is always flowing to the tub spout and relies on the spout diverter gate to be raised to go back up to the shower, wouldn't a reduced flow to the shower create back pressure to the tub spout to cause more water to splash out of the spout diverter gate too?

Sum, on a 2 outlet port shower valve without a diverter, the top and bottom ports freely communicate, so a restriction to the tub spout will allow more water to back up through the valve to the top port. A restriction in the feed to a twin ell will reduce flow to both the tub and the shower, so the shower will not be subject to back up. I don't see a PEX restriction in shower feed, from the twin ell, affecting the spout diverter leakage.

steve, thank you. so just to make sure I understand, a four port valve (2 ports left and right for feed, one top port for shower and one bottom port for tub), the top and bottom ports stay open.

So the design is:

for shower only installation, plug the bottom port, use the top port only, tubing material doesn't matter.

for tub spout only installation, plug the top port, use the bottom port only, tubing material doesn't matter unless of course, filling the tub faster with copper slower with PEX.

for shower/tub combination with a diverter valve, plug one port, use the other port to feed the diverter valve, and from the diverter valve has two outlet one to shower one to tub, and since the diverter will close one port and open the other, piping material doesn't matter.

for shower/tub combination with no diverter valve, but only a tub spout diverter gate, if you use the top port for to shower and the bottom port for tub, this is where PEX has an issue, the restriction of PEX reduces flow to tub spout and causes the water to back up and rise higher in the shower tubing and spill out there. Using PEX A or copper between valve and tub solves the problem.

for shower/tub combination using a twin-el, you would close off the top port, run a pipe A from bottom port to the twin-el, then a pipe B from twin-el up the wall to the shower, and a short pipe C from the twin-el out to the tub spout. In this case, pipe A and C must not be PEX, pipe B can be PEX. If either A or C is PEX, then a restriction happen and the shower will spill when the tub spout is on. Pipe B from twin-el to shower can be PEX doesn't make a difference.

am I understanding this right?

Correct. Using Pex for pipe C would wrong for a couple of reasons.

Oh yes you are right, using PEX for pipe C would be like a tub spout with a garden hose!