A backflow preventer is an assembly, device, or method that prevents the reversal of the flow of liquids into a potable water supply system. Local utilities departments require testing of the backflow assemblies to ensure they are working properly. Depending on the County/City you live in, you will be required to have the backflow tested annually, biennially, or decennially. If you are unsure which category you fall under, please contact your utilities department.