We've pointed out the effectiveness of thermostatic valves previously on TreeHugger. These devices mix the hot and cold water of your shower, so that the output water stays at a fixed temperature. They are different from pressure balancing valves. These valves regulate the pressure of hot water to match the pressure changes of the cold water, thus indirectly keeping outflow temperature more or less constant. So you don't get scalded in the shower when someone flushes a toilet. One drawback is that they are not as accurate at maintaining a constant temperature as thermostatic valves.Anti-scalding devices are required by code in new buildings in California and Canada. They also save on water and water heating, because you don't spend as much time adjusting to get the right temperature in the shower. These values are available in most hardware stores now. Some water heaters also have anti-scaling devices built-in too. :: Plumbing Supply Anti-Scalding Devices.