Antiscald, also called pressure-balance, valves are designed to prevent sudden blasts of hot or cold water coming through the showerhead when another plumbing fixture is turned on or the toilet is flushed. A jolt like this poses a very real hazard, because it can cause you to slip and injure yourself. Burns are another danger, especially for children and older folks who have sensitive skin.
Antiscald valves work by detecting and compensating for changes in pres sure in the hot and cold supply lines. Even better, you can set the maximum temperature for an individual valve (see "Adjusting an Antiscald Valve" image below).
Most antiscald valves have a single handle; however, many older homes have two- or three-handled valves. In this situation, you can either look for a two-handled antiscald valve or special order an escutcheon plate that will cover the old handle holes.
To avoid or minimize tile repairs, install the valve from the back side of the tub or shower wall and tie into the existing copper pipe. If you have galvanized pipe, you can transition from galvanized to copper by using a dielectric union to prevent corrosion.
Protect your tub and shower and keep debris out of the drain by taping an old sheet to the tile wall below the work area. When dry-fitting pipes, make sure all the joints seat completely so the measurements will be accurate. Finally, install the shutoff valves right away so you can turn the water back on to the rest of the house.
1. Remove handle and escutcheon plate from valve. Shut off water supply to shower. Measure setback from tile surface to center of pipes in wall. To simplify installation, buy a valve with a similar setback.
2. Cut an access hole behind the valve if you don't already have one. Remove tub spout and shower arm and cut supply pipes where shut off valves will be located. Remove old faucet valve and all old pipe above shutoff cuts.
3. Use old valve and pipes as a template for sizing new valve assembly. Solder connections. Use a fireproof cloth to protect flammable materials in the work area. Open the shutoff valves and remove valve cartridge while soldering.
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4. Secure valve and pipes with support blocks and pipe straps (see
Step 3). Install escutcheon plate, set antiscald limiter and install
handle. Install tub spout and shower arm and test the valve and connections. 
Adjusting an Antiscald Valve
Antiscald valves usually have a gear- like rotational stop behind the handle that controls the hot water flow by limiting how far the handle can be turned in the "hot" direction. Just remove the handle to gain access.
Safety: Not so hot!
If your home isn't equipped with antiscald valves, lower the water heater setting so the maximum hot water temperature is around 1200 F (50° C). Use a cooking thermometer to check the temperature.
