The answer to 'when to replace backflow preventer' is that backflow preventers should be replaced every 5-10 years, or sooner if they are not functioning properly. Backflow preventers are critical devices that prevent contaminated water from flowing back into the clean water supply, so it's important to ensure they are in good working condition.
Over time, the internal components of a backflow preventer can wear out or become damaged, compromising its ability to prevent backflow. Signs that a backflow preventer needs to be replaced include leaks, reduced water pressure, or failing the annual inspection and testing required by most local plumbing codes. If you notice any issues with your backflow preventer, it's best to have a licensed plumber inspect and replace it as needed to protect your home and the public water supply.