Begin by ensuring a safe and prepared work area. Locate the main water shut-off valve for the house and turn it completely off. To drain the remaining water from the system, open the faucet at the lowest point in the house (like a basement sink) and the shower faucet you are replacing. Lay down a drop cloth in the tub or shower base to protect the surface from scratches and debris. Gather all necessary tools and materials, and put on your safety glasses and work gloves.

To access the valve, you must create an opening in the wall. First, remove the old handles and trim plate (escutcheon). Use a stud finder to locate the studs on either side of the valve. Mark a rectangular cutting area around the valve that is large enough to work in, typically about 12x12 inches. If working with tile, use an angle grinder with a diamond blade to carefully cut along your marked lines. Go slow to avoid cracking adjacent tiles. Once the tile and underlying backer board are cut, use a pry bar to gently remove the section and expose the old valve and surrounding pipes.

With the old valve exposed, you can now remove it. Identify the hot supply, cold supply, shower riser (to the shower head), and tub spout pipes. Using a tubing cutter, make clean, straight cuts on all four copper pipes connected to the valve body. Ensure you leave enough pipe extending from the wall (at least a few inches) to connect the new valve. Once all pipes are cut, the old valve body should be free to be removed from the wall cavity.

Proper pipe preparation is essential for a leak-free solder joint. Take a deburring tool and insert it into the end of each cut pipe, rotating it to remove the sharp inner burr created by the cutter. Then, use sand cloth or an emery cloth to thoroughly clean and polish the outside of the pipe ends (about 1 inch back from the end) and the inside of the new copper fittings until the copper is bright and shiny. This removes any oxidation and surface contaminants that could prevent the solder from flowing properly.

Before soldering, remove the plastic cartridges from the new valve body to prevent them from melting. Dry-fit the valve with short pieces of copper pipe and fittings (like couplings) to ensure everything lines up. Once satisfied, disassemble it. Using a brush, apply a thin, even coat of flux to the cleaned exterior of the pipe ends and the cleaned interior of the fittings. Reassemble the joints. Place your heat shield behind the work area to protect the wood studs. Using a propane torch, heat the fitting, not the pipe. When the flux sizzles and the copper changes color slightly, touch the lead-free solder to the edge of the joint. The heat from the fitting will melt the solder and capillary action will draw it into the joint, creating a complete seal. You will see a silver ring appear around the seam. Repeat for all connections.

Before closing the wall, the valve must be secured. Install a 2x4 wood brace (stringer) horizontally between the wall studs, positioning it so you can anchor the valve body to it. Use a level and tape measure to ensure the valve is plumb and set at the correct depth from the finished wall surface (check manufacturer specs for this dimension). Secure the valve to the brace with screws. Once the solder has cooled completely, reinstall the valve cartridges. Go to the main water supply and turn it back on VERY SLOWLY. Return to the shower and meticulously inspect every solder joint for any sign of a leak or drip. Use a dry paper towel to wipe each joint, as this will reveal even the tiniest amount of moisture.

After confirming there are no leaks and the wall has been repaired (a separate process of installing backer board, tile, and grout), you can install the final trim. Slide the escutcheon plates over the valve stems and onto the wall. Apply a bead of silicone caulk around the top and sides of the escutcheons where they meet the tile to create a watertight seal, leaving the bottom unsealed to allow any moisture to escape. Attach the handles to the valve stems and tighten their set screws using an Allen key. Screw on the new tub spout and shower head, using Teflon tape on the threads of the pipe nipples to ensure a good seal.