A back flow preventer is a check valve that prevents water from flowing backwards into a water supply line resulting in a contaminated water supply. There are degrees of protection ranging from a vacuum breaker on a garden hose line to reduced pressure zone backflow preventers commonly used in an industrial setting where chemicals are connected to water lines and if pressure was lost on the water supply cross contamination could occur. What is used and where it is installed depends on the application and the treat level. Call Knights Plumbing and we'll get started right away. We have several certifiers on staff and we'll submit all the paperwork for you.