Many homes and commercial buildings in the 5 Boroughs have a backflow prevention valve installed.
These devices are designed to prevent mixing between contaminated water sources and the municipal clean water supply. Usually installed in areas where those pipes might cross-connect.
Backflow preventer valves are commonly used on properties that have large boilers, fire suppression systems, or irrigation systems, which tend to create an elevated risk of backflow. Backflow itself results from pressure differences, causing water to move in the wrong direction and contaminate the potable supply.