How To Install an Anti-Scald Shower Valve

Anti-scald shower valves are for safety and code compliance just as much as they are for comfort. Sometimes called pressure-balance valves, these devices use an internal piston that moves with changing water pressure to keep the temperature steady and prevent nasty burns. This Old House plumbing and heating expert Richard Trethewey demonstrates how to replace an outdated three-handle valve with a single-handle anti-scald valve.

Preparing for Installation

Before beginning the installation process, you’ll need to gather the necessary materials and take a few safety precautions.

Tools and Materials Needed

• Cold chisel (1/2-inch)
• Cordless drill/driver
• Cover plates (to go over holes left in the wall from old valves)
• Digital thermometer
• Faucet handle puller
• Hammer
• New anti-scald valve
• Oscillating multi-tool
• Pipe cutter
• Pipe wrench
• Pliers
• Propane torch
• Reciprocating saw
• Screwdriver
• Wet/dry vacuum

Safety Precautions

Before starting the installation, follow these safety steps:

• Cover the drain so small parts can’t fall in.
• Have a fire extinguisher nearby in case soldering work goes wrong.
• Turn off the water supply to the shower.
• Wear safety glasses and work gloves.

Removing the Old Valve

Trethewey begins by removing the shower valve, inside and out.

Removing Handles and Trim

First, unscrew the handles and chrome escutcheon plates from the existing shower valve. The old valves in Trethewey’s project have screws at the center of the buttons that hold the handles in place. “Sometimes, these can be miserable if they’ve been in for a long time,” he says. If buildup or tension makes the handles hard to pull, use a handle puller, as demonstrated in the video. Once the handle is off, the escutcheon plates can slide off the stem units.

The old valves also have three stem units to remove: one for hot water, one for cold, and one diverter used to cycle between the tub and shower. You can remove them using a pipe wrench.

Removing the Old Valve’s Internal Components

Next, you must cut the wall open to reveal the valve body. If you’re using a large cover plate for the new valve, it may come with a template you can mark and trace with.

Start your cut by drilling a few strategically placed holes with a masonry bit. Then, use an oscillating saw with a carbide-grit blade to cut along the marked lines and between each hole. A hammer and chisel clear away the old tile.

Once the tile is cut and moved away, Trethewey uses a reciprocating saw to cut the piping for the hot and cold water valves away, leaving only the piping for the diverter. Don’t go in too deep with the saw, or you may risk accidentally cutting into a supply pipe.

Installing the New Anti-Scald Valve

With the old valve removed, it’s time to install the new anti-scald valve. You’ll solder the connections together and install a cover plate to make everything look nice.

Before you start, stick the stem unit for the new valve onto the diverter pipe to make sure it aligns with your existing plumbing. Then, clean the pipes and fittings for better performance.

Soldering Connections

Apply flux to all copper surfaces you’ll be soldering. Thoroughly dampen the surrounding area with a spray bottle or plant mister to lessen the fire risk. Remove the spindle from the new valve to prevent damage from heat. Solder all the connections carefully for a watertight seal.

Testing the Installation

Allow the metal to cool down, then reinstall the valve spindle and turn the water supply back on. If you see no leaks inside the valve box, that’s a good sign everything is tightly sealed.

Then, test the valve’s operation by flipping on your tub. Turn the temperature from cold to lukewarm to hot, and continue to check for leaks.

Finishing Touches

Once the new valve passes testing, it’s time to install the cover plate and trim pieces. 

Installing the Escutcheon Plate

Apply some plumber’s putty to the back of the cover plate and screw it in place. The cover plate Trethewey uses has metal spanners that further tighten the plate from the other side as he tightens the screws.

Next, install the escutcheon plate with indicator dials. Line it up with the stem and secure the screws according to the manufacturer’s instructions. There may also be a gasket you have to install that stops water from coming through the edges of the plate.

Setting the Temperature Limit

Before you install any more trim, adjust the valve’s safety limit—the maximum water temperature that can come out of the shower and tub faucet. Any water over 120 degrees Fahrenheit can scald, so don’t leave the limit set any higher.

Dry-fit the handle to the stem for leverage and turn the water on to its hottest setting. Measure the temperature with a digital thermometer. If the water is too hot, turn it off, then tighten the temperature limit screw at the base of the stem. Continue to test and tighten until the water reaches 120 degrees Fahrenheit or cooler.

Once you’ve calibrated the temperature limit, install the handle and any other remaining trim. Clean up any stray plumber’s putty with a rag.