HOW TO REPLACE EMBEDDED PEX FITTINGS

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Step 1: Removing the tiles around the PEX fittings

Corroded outlets leak behind tiles and infiltrate lower ceiling.
Patiently get the rid of the first tile.
These aged 20 years and need to be replaced.
Now you can see the mortar that was used to fix it.
Start by clearing the joint with a flat screwdriver.
Use a flat chisel to break the mortar with a 2 Kg hammer.
And breaking the tile little by little. Wear safety goggles !
Get the rid of it. Take care not to damage the bath tub.
Change to a point chisel and start freeing the embedded water outlets.

Step 2: Removing the corroded PEX compression elbow fittings

Little by little the embedded elbow emerges.
Try all screwdrivers you have as the screw heads will be completely corroded.
Make sure it is freed all around.
Hit them counter clockwise with a flat screwdriver.
If you need to break a bit of the tiles, use a tile cutter.
Whenever you can, unscrew them with grip pliers.
The wall outlet is now ready for the next step.
You can consider yourself lucky if you succeed!
If the screws don't move, simply drill them !

Step 3: Pulling the PEX tube out of the PEX manifold

After releasing the PEX tube from the manifold, pull it out using the corroded elbow as a handle.
Keep pulling steadily alternating your hands.
The extents of the corrosion are well visible on this PEX connector.
Now continue removing the screws which did not unscrew. Bend them and cut them with grip pliers:

Step 4: Pull new PEX tube, fit the PEX elbow connectors

Do not reuse the same tube. Buy a few meters.
Introduce the new PEX and push steadily and firmly...
...until it shows at the other side.
Use a screwdriver to pull it out towards you.
Now start by making a U for each outlet using 2 compression elbows.
Using a cutter, make a chamfer prior to introducing the elbow.
Be careful not to damage the seals. 
Use one drop of dish-washing liquid as a lubricant.

Step 5: Install surface mount PEX wall fittings

Surface mount outlets are used as shown here. 
Temporarily fix the outlets with screws.
Hold the outlet and mark the holes.
Measure the length of the small tube.
Make sure your marks are well visible.
Unscrew the outlet, fit the tube and fix again the elbow to the wall.
Drill them and fit wall plugs.
Note: don't forget that the height of the wall outlet is the one of the shower mixer. Make sure such height is convenient and that both outlets are fixed at the same level. Also consider fixing your outlets at the same spacing than the existing ones.

Step 6: Make a leak test and prepare for the shower mixer installation

Both outlets have now been fixed.
Open the water tap and let it flow for a few seconds.
Temporarily plug the outlets then open the taps again and leave them for a couple of hours as a leak test.
Now proceed as shown for temporarily fixing the mixer.
Cover the 1/2" thread of the eccentric with one layer of Teflon tape.
Add one layer of plumbing hemp. It will help blocking the rotation of the eccentric.
Cover the hemp with a thick layer of Teflon tape.
Mount the eccentric into the wall outlet and adjust it as shown below.
Measure the distance between your outlets after each adjustment until it matches the one of your shower mixer. Try the mixer by screwing it in by hand to make sure it fits well then remove it.

Step 7: Time to cover up the broken tiles

There are several ways of repairing or covering the broken tiles:
  1. Re-tile but then you face the same problem upon the next repair with the need to break the tiles and the concrete
  2. Install a marble plate 
  3. Install an acrylic plate (Plexiglas)
My choice is for a 10 mm thick acrylic plate which is shown in the below pictorial.
Start by taking the necessary measurements. 
Now drill the openings in the Plexiglas plate.
Fix the plate with silicone sealant. Note well the spacers that have been used to keep the panel a bit away from the wall.
Complete the installation by fixing the mixer. Wait 24 hours before using it.
The job is complete, elegant, practical as it allows to intervene in the future without having to break tiles.