Backflow preventer
A valve that stops dirty water from flowing back into your clean supply.
A backflow preventer is a one-way valve that keeps used or contaminated water — from irrigation, a hose, or a boiler — from being siphoned back into your drinking-water lines if pressure drops. It protects your home's water from contamination. Outdoor spigots and irrigation systems often have them, and some jurisdictions require periodic testing.