Those things can be fickle.

Sometimes it can be caused by insufficient head between the shower head and the valve/tub spout. Meaning some rely on gravity to prevent the water from flowing up the vertical pipe and exiting out the shower head.

If you mounted the shower head lower than recommended, or vice-versa, mounted the valve/tub spout higher, that could be a problem.

Restrictions in the faucet supply tubing, as spacechallenged wrote, can be factor. If there is a restriction in the faucet plumbing then the water can back up and exit the shower head.

Too high water pressure. Most residential plumbing runs around 40-45psi or so. If on city/town water and your pressure s higher and you don't have a pressure-reducing valve, high supply pressure can cause it as well.

I'll assume that the plumbing was done correctly and the plumbing flushed so there is no debris in the system?

Is your diverter on the valve itself or on the tub spout?

If you remove the tub faucet and just leave the tub nipple and turn the water on, does water still back up out of the shower head?

Does water come out of the shower head at all volume levels or just high levels?

Sounds to me like a few things you both mentioned could be the problem.

1. We do have PEX everywhere.

2. We have a constant pressure system built on our well.

3. I think we put the shower head a little bit lower for the kids.

Yes the shower leaks at all settings low and high. He did remove the tub faucet and it still comes out the shower head.

What do you think should be the first thing we do to try to fix this problem. Space mentioned something about changing out the PEX to copper but how do you do that now since the valve was built into the studs and drywall and the tub surround are over both now. It's so frusterating to have something brand new and not have it work properly.

I will consider all help suggestions. Thank you

First thing I'd do is read the installation instructions to see what is specified there. If they recommend "X" number if vertical inches and you're less than that, or if you used PEX and they specify "no pex" or you used pex that has a smaller inside diameter than the recommended tubing, then essentially you have an installation issue. 1/2" pex does have a smaller inside diameter than 1/2" copper. For delivering water volume to a faucet it's normally not a problem, but when depending on a specified vertical water column, I guess the smaller diameter of the pex could be an issue.

I'd call the manufacturer's tech support line and ask them about remediation. No harm no foul, it's best to run ideas past them before you crack open walls.

You installed the shower head lower for the kids. Let's say you have yours 25" above the valve and the manufacturer specifies 30". What you could do is have the pipe come vertically out of the valve, and run it up the wall several inches higher than the shower head, say to 40", or even all the way to the top plate of the wall, then add a couple of 90's (to create and inverted "U") and then run it back down to the shower head. That would take care of the vertical issue while allowing you to still have a lowered shower head.

If your pex to the tub spout is constricting flow out of the tub spout, that might need to be changed to a larger size of PEX or to copper. Something with a larger inside diameter.

You're on your own well so it's likely excessive water pressure isn't an issue.

What's on the "back" side of the wall that the valve and plumbing is installed in? Often times it's easier to cut through the back of the wall to access the plumbing, repair the plumbing, then TEST the plumbing, then patch the hole, than it is to access through the shower side of the wall.

In my case I took pics throughout, so we easily identified the problem. We were able to cut through the back side which was just drywall and the plumber fixed it replacing that short section of pex with a brass fitting. The problem as mongoct says, is the pex is a smaller diameter and causes a restriction in flow. You could go with copper, brass or galvanized fittings from the valve down to the tub spout. The pex that went from the valve up to the showerhead was fine and not causing any problem in our case. As mongoct says, do whatever the manufacturer recommends. Are you able to remove the tub spout and valve and shine a flashlight to see if there is pex from the valve to the tub spout? The new pex is usually blue or red.

dreamywhite,
I know how stressful it can be to go through all that you go through during a remodel and have something go wrong like that.
It seems like alot of the time I was taking 1 step forward and 2 back.
If it's the pex causing the problem or if it is something else, I'm hoping it will be easily fixed. Best of luck.

We have the same problem right after a shower remodel. The installer uses Pex routinely and he did elevate the shower outlet higher than it was, but still have this issue. I already fired the contractor for other reasons but it is good to know now before we let someone else do a similar remodel on our master bath….but that only has a standup shower, so would not be a problem. I will have to think of another way to solve this as the back side of the wall had already been cut out once, by the original remodel contractor because of a mistake he found. What a joke that guy is.

I have the same problem. I'm debating putting shutoff valves on the water supply lines to reduce the water pressure. Not sure it would work, but worth trying before I go cutting into walls (my supply is accessible in the basement).

We have the same issue in a brand new shower valve with diverter done during a remodel. The plumber has replaced the diverter twice and the shower still leaks every time you turn on the tub. They used copper pipes and the shower is mounted high. do you think pressure is the issue? we barely get enough pressure to get our tankless water heater to turn on so i dont know what we would do if that is the issue. any advice welcome!!!