Which device prevents backflow into the water supply?

• A. Expansion tank
• B. Automatic air vent
• C. Backflow preventer
• D. Flow check

Answer: (C)

Backflow into the water supply is prevented by a device designed specifically to stop reverse flow and protect clean water from contamination. A backflow preventer is built to block any backflow that could occur due to pressure changes, back-siphonage, or back-pressure, ensuring water can only move away from the supply and into the building’s systems, not back into the potable supply. It often includes multiple valves and sometimes a vent or relief mechanism to handle different backflow scenarios, meeting code requirements for cross-connections.

The other devices don’t provide that same level of protection. An expansion tank mainly absorbs excess pressure from thermal expansion and helps prevent water hammer; it isn’t a barrier to backflow. An automatic air vent simply removes trapped air from hydronic systems. A flow check (check valve) lets water move in one direction but may not offer the comprehensive protection or the required safeguards for potable water systems, especially during complex pressure fluctuations, so it isn’t the device relied on for preventing backflow into the water supply.