The answer to 'why is my backflow preventer rusty water' is that a rusty backflow preventer is usually caused by corrosion and mineral buildup over time. This can happen when the backflow preventer is exposed to water that is high in minerals, chemicals, or acidity. The rust can indicate that the backflow preventer is failing and needs to be repaired or replaced.
Rusty water coming from the backflow preventer is a symptom that the internal components are deteriorating. This can allow contaminants to potentially backflow into your home's water supply, which is a serious health hazard. It's important to address a rusty backflow preventer promptly to prevent further issues.