Which feature provides a physical separation between a water outlet and a potential contamination source to prevent backflow?

• A. Air gap
• B. Check valve
• C. Backflow preventer
• D. Float valve

Answer: (A)

An air gap creates a physical separation between where the water comes out and any source that could be contaminated, using open air as the barrier. This breaks the hydraulic connection entirely, so water can’t be siphoned or pushed back into the potable supply even if downstream pressures change or contamination is present.

This works because there’s no direct pipe-to-pipe connection allowing backflow—only air sits between the outlet and the contaminate source. In practice, you might see an air gap used with dishwashers or certain faucet installations, where a small vertical gap ensures any return flow would have to travel through air first, not back into the water line.

Other options are devices that try to control flow or block it through the pipe, but they aren’t true air gaps. A check valve prevents backflow by closing the path when pressure reverses but can fail or be bypassed under certain conditions. A backflow preventer is a device designed to block backflow, yet it still relies on proper installation and maintenance and doesn’t provide the same guaranteed atmospheric separation as an air gap. A float valve is about stopping water at a certain level in a tank and doesn’t address backflow into the potable supply.

So, the air gap is the feature that provides that physical separation needed to prevent backflow.