The answer to 'why is my backflow preventer constantly running' is that the backflow preventer is likely malfunctioning. A constantly running backflow preventer is usually a sign of a problem, such as a faulty check valve, a stuck diaphragm, or a problem with the internal components. This can lead to water pressure issues, water waste, and potential water damage.
If your backflow preventer is constantly running, you'll want to inspect it for any visible signs of damage or wear. You may also want to perform a backflow test to identify the specific issue. In some cases, the backflow preventer may simply need to be cleaned or adjusted. However, if the internal components are damaged, you'll likely need to replace the entire unit. This is a job best handled by a licensed plumber, as backflow preventers are complex devices that require specialized knowledge and tools to repair or replace properly.