The answer to 'backflow preventer constantly running' is that the backflow preventer is likely stuck in the open position, preventing water from flowing back into the main supply line. This can be caused by debris buildup, a faulty valve, or a mechanical issue with the device.
Common symptoms include constant water flow, high water bills, and potential contamination of the main water supply. Solutions may involve cleaning or replacing internal components, or in some cases, replacing the entire backflow preventer unit. If the issue persists or you're unsure of the cause, it's best to call a licensed plumber to diagnose and repair the problem properly.