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YIPES! 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. Sound familiar? If your shower or bath water's temperature fluctuates widely with water use elsewhere in the house, you're living with a serious hazard.

The Consumer Product Safety Commission (CPSC) estimates that 200,000 bathroom injuries occur each year, frequently the result of cold- and hot-water shocks and scalding. Startling thermal shocks from cold or hot water can trigger serious falls, particularly with older or physically challenged people. And, because children have thinner skin than adults, they are especially vulnerable to scald burns from overly-hot bath or shower water.

About 5000 kids 14 and under are scalded in the bath each year. As William Kamela, Director of Public Policy for The National Safe Kids Campaign, points out, "It takes just 3 seconds for a child to sustain a third degree burn from water at 140 degrees Fahrenheit." Tap water temperatures in homes and public buildings commonly reach from 140 to 160 degrees F.

Kamela advises that one of the simplest scald-prevention measures you can take is to lower the water heater thermostat to 120 degrees. (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.)

But that won't solve the problem of temperature fluctuations. The way to elminate those shifts is to install pressure balance, anti-scald valves at tub and shower locations. "These maintain water temperature at a safe level, despite fluctuations in supply lines," Kamela adds.

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 may reach the shower valve, changing the shower's 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-balance tub or 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, depending upon the manufacturer. Most reduce water flow to a trickle if the cold water supply fails.

For additional scald protection, most anti-scald valves have a stop that prevents the handle from rotating beyond a set position, limiting the amount of hot water the valve can deliver. This stop, a ring or adjustment screw, is set at the maximum desired temperature--usually about 120 degrees--by the installer or homeowner. Some models have a variety of pre-selected temperatures; the KWCdomo fixtures, for example, offer seven preset temperatures.

Some valves adjust water to the proper temperature for you. American Standard's CeraTherm(tm), for example, lets you select water temperature on a dial and then automatically keeps it there. For added safety, the hot water control won't rotate beyond 100 degrees F unless you press a red safety button. Delta's deluxe Monitor 1500 Series faucets also have a feature that automatically adjusts water to a previously-set temperature.

A pressure-balance valve is installed in the same place as a conventional bathtub or shower valve--generally in the wall, immediately behind the control trim. A valve may control only the shower or tub spout or, if it's a "diverter valve," both tub and shower.

According to Delta's president, Chuck Dowd, "The pressure-balance valve category of faucets has grown 30 per cent annually since 1990." More than 70 percent of all tub and shower valves sold in the United States are now anti-scald.

The popularity of these valves is driven by a growing number of states adopting codes that require these products. They are currently mandated by law for new construction in 28 states and 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 shower heads. Again-- these new requirements affect only new construction.

Can you replace an existing valve with a scald-free model? You bet; the difficulty depends on the existing valve. A plumber can usually replace a single-handle, conventional valve with a pressure-balance 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; it may call for removing some tile or cutting into the wall. Several valve manufacturers sell "remodel plates" to cover-up the cut-out area.

Because nearly all faucet manufacturers now offer scald-protection valves, prices run the gamut, depending on the styling and finish of the trim. Do-it-yourself retailers offer packaged tub-shower pressure-balance valves for as little as $80. At the other end of the spectrum, you can pay $700 or more.

--Don Vandervort