The answer to 'my backflow preventer smells bad' is that a foul odor coming from your backflow preventer is often caused by a buildup of debris, bacteria, or mineral deposits. This can happen over time as the device functions to prevent contaminated water from flowing back into your home's clean water supply.
Some common symptoms of a smelly backflow preventer include a rotten egg or sewage-like odor, and the smell may get worse when the device is in use. To solve the issue, you'll typically need to clean or service the backflow preventer. This is usually a job best left to a professional plumber, as improper maintenance can lead to serious water contamination issues.