It's happened to everyone who showers: Elsewhere in the house a toilet is flushed, a faucet is opened, or the washing machine kicks on, and the once-temperate water coming from the showerhead is suddenly cold enough to make you jump or so hot you want to scream. A simple device called a pressure-balancing shower valve can help.

By adjusting to pressure changes in water coming through the hot and cold supply lines, a piston in the valve automatically opens or closes small inlet ports to maintain a balance in pressure, which in turn keeps the water flowing at an ambient temperature. "It reacts instantaneously," says Bill Tracey, marketing manager of Symmons Industries, whose founder invented the prototype in 1938.

Pressure-balancing valves are now required in new residential construction in most states, depending on local plumbing codes. In addition to piston valves like the one shown above, some pressure valves use a wheellike diaphragm to trigger pistons that cover the hot-or cold-water ports. Both are priced at around $70, not including installation, which is a bigger issue with an existing shower. It involves cutting through the wall behind the shower, removing the old valve, and soldering on the new one. But it's a project that This Old House plumbing and heating expert Richard Trethewey strongly recommends. "There's no downside," he says. "One of these valves will really keep you out of hot water. Or cold, as the case may be."