Here’s the typical scenario: Just as you’re enjoying a nice, warm shower, a blast of icy cold water races shivers down your spine. Or, worse, somebody flushes a toilet and the shower water sizzles your skin.

Startling thermal shocks from cold or hot water can trigger serious falls, particularly in older or physically challenged people. And, because children have thinner skin than adults, they are especially vulnerable to scald burns from hot water.

The way to eliminate those temperature shifts is to install a pressure-balanced anti-scald valve or thermostatic temperature-control valve in the shower wall where the shower controls are located.

A valve may control only the shower or the tub spout or, if it is a diverter valve, both tub and shower. These maintain water temperature at a safe level, despite fluctuations in water supply lines. (One of the simplest scald-prevention measures you can take is to lower the water heater thermostat to 120 degrees F. To check your hot water’s temperature, place a meat thermometer in a glass and run hot water into it for a couple of minutes.)

The problem that causes temperature fluctuations is basic. When a toilet is flushed, cold water flows into the tank to refill it, causing the water pressure in the cold water pipes to dip. If this happens when you’re showering, less cold water reaches the shower valve, changing the comfortable hot/cold mix to hot only. If someone turns on a hot water faucet elsewhere in the house, the opposite can happen—the hot water drops and you get a shot of cold. This problem is exacerbated in plumbing that’s clogged with mineral deposits, in relatively small (1/2-inch or smaller) supply piping, and in showers with low-flow showerheads.

A pressure-balanced shower valve is designed to compensate for changes in water pressure. Though it looks like any other shower or tub valve from the outside, it has a special diaphragm or piston mechanism inside that moves with a change in water pressure to immediately balance the pressure of the hot and cold water inputs. These valves keep water temperature constant, within plus or minus 2 to 3 degrees F., by reducing water flow through either the hot or cold supply. Most reduce water flow to a trickle if the cold water supply fails.

Flow can be a problem with pressure-balanced valves, particularly in homes where the shower includes a personal hand shower and/or a multiple-head shower system. Most pressure-balanced valves are either full-on or full-off.

Where flow and volume control are important, a better choice is a thermostatic valve. Most of these have 3/4-inch inlets that can blast a flood of water through multiple showerheads and will maintain the water temperature within 1 or 2 degrees F. of the set temperature.

“Choose a thermostatic over pressure-balanced valve if you want maximum flow and volume control,” says a spokesperson for George’s Pipe & Supply in Pasadena, California. ”A thermostatic valve will allow you to maintain a set temperature when you turn the valve off and on if you like to soap up and then rinse.” Of course, you’ll pay for the difference. Whereas a pressure-balanced valve will typically cost from $100 to $250, a thermostatic valve with a volume control (a separate feature with most), will run from $400 to $1,000 depending on the features and trim.

The popularity of these valves is driven by codes that require these products in new construction. In addition, all major plumbing certification and code organizations have adopted anti-scald requirements that call for plumbing fixtures to have a built-in means of controlling maximum water temperature delivered through tub spouts and showerheads.

Can you replace an existing valve with a scald-free model? You can, but the difficulty depends on the existing valve. A plumber can usually replace a single-handle, conventional valve with a pressure-balanced valve after removing the handle’s trim (if there isn’t access from behind the wall). Replacing a dual-handle setup is a little more involved and may call for removing some tile or cutting into the wall. Several valve manufacturers sell “remodel plates” to cover up the cut-out area.