A RPZ valve is a valve designed to prevent water from flowing backward into the inner reaches of a pipe system by diverting water along an alternate course if a sudden change in pressure is detected. The water, rather than backing up into the pipe, is drained out.
RPZ stands for reduced pressure zone. If a pipe contains a region that has unusually low pressure, its contents can get sucked back into this section. For example, when a toilet is flushed, there may be a part of the pipe that suddenly has reduced pressure due to a large amount of water emptying out. Any remaining water at the front end of the pipe will get sucked backward into the low-pressure region, like a beverage through a drinking straw.
Although the toilet example is relatively innocuous, bigger trouble can occur when municipal water supplies experience similar reductions in pressure: Waste water can back up into drinking water supplies. RPZ valves can detect the change in pressure and dump out water that is pushing backward towards outlets, thereby preventing contamination. They are used in pipe systems that need extra backflow protection to prevent illness, such as hospitals or commercial kitchens.