The answer to 'when to replace backflow preventer' is that backflow preventers should typically be replaced every 5-10 years, or whenever they fail a required annual inspection and testing. Backflow preventers are essential devices that prevent contaminated water from flowing back into the clean water supply, so it's important to ensure they are functioning properly.
Over time, the internal components of a backflow preventer can wear down and become less effective at preventing backflow. Annual testing is required by most local plumbing codes to verify the device is working correctly. If the backflow preventer fails the test, it needs to be repaired or replaced immediately to protect the home's water supply.