How to Flush a Shower Valve

Purpose

Flushing removes debris, mineral buildup, or old lubricant from the water lines and valve body before installing a new cartridge. This prevents damage to the new cartridge and ensures smooth operation.

Tools and Materials Needed

Rags or towels
Small bucket or bowl (optional but recommended)
Paper or plastic cup with the bottom cut out (or a dedicated flushing tool if available)
Adjustable pliers or cartridge puller tool (if needed)

Step-by-Step Instructions

Turn off the water supply to the shower (hot and cold shut-off valves if present, or the main water supply to the house).
Open the shower faucet (handle or lever) to relieve any remaining pressure and drain residual water.
Protect the wall and surrounding area
Stuff a clean rag or towel into the wall cavity directly below and around the valve opening. This catches water and prevents it from running inside the wall and causing damage.
Remove the old cartridge
Follow the manufacturer’s instructions for your specific valve (usually involves removing the handle, trim plate, and using a cartridge puller or pliers). Place the old cartridge aside.
Prepare a flushing shield (highly recommended)
Take a disposable paper or plastic cup (8–12 oz works well) and carefully cut out the bottom with scissors or a utility knife, creating a bottomless tube.
This “cup” will act as a shield to direct water straight into your shower/tub instead of spraying everywhere.
Insert the flushing shield
Slide the bottomless cup into the valve cavity so it surrounds the opening where the cartridge was. It should fit snugly and extend outward 2–4 inches. Hold it in place or have someone assist.
Flush the lines one at a time
- Slowly turn on the cold water supply only (keep hot fully off).
- Let water run forcefully for 30–60 seconds or until it runs clear.
- Turn cold water off completely.
- Repeat the exact process with the hot water supply only (30–60 seconds or until clear).
Remove the flushing shield and rag
Carefully pull out the cup and rag. Have a towel ready—some water will still drip.
Inspect and dry
Wipe the valve cavity dry with a rag. Check for any remaining debris and remove it.
Install the new cartridge
Follow manufacturer instructions exactly (lubricate O-rings if required, align properly, etc.).

Tips and Warnings

Water can come out with surprising force—be prepared for splashing even with the cup shield.
If you don’t have a cup, you can try holding a rag tightly around the valve opening, but the bottomless cup method is far cleaner and more effective.
Always flush one side at a time—never open both hot and cold simultaneously while the cartridge is removed (this can cause cross-flow and contamination).
Wear safety glasses; debris can shoot out.

By following these steps, you’ll keep water out of the walls, protect your bathroom, and greatly extend the life of your new shower cartridge.