Which backflow device primarily protects against back-siphonage?

• A. Double-check valve, which primarily protects against back-siphonage.
• B. Reduced pressure principle backflow preventer, which protects only against backflow.
• C. Air gap, which prevents backflow by distance.
• D. Gate valve, which is not a backflow preventer.

Answer: (A)

Backflow prevention is about stopping contaminants from being drawn back into the potable supply when pressure in the system changes. Back-siphonage happens when supply pressure drops, creating a vacuum that can pull water backward through cross-connections. The double-check valve is designed specifically to block that siphoning effect. It has two check valves in series with a small vented chamber between them; if pressure reverses, the valves close and the vent helps prevent a siphon from forming, so water cannot flow back toward the supply. That focus on stopping siphonage is why this device is the best choice for preventing back-siphonage.

An air gap prevents backflow by creating a physical distance between the supply and the downstream system, which is an effective barrier, but it relies on proper installation and space rather than a valve mechanism. The reduced pressure principle backflow preventer provides strong protection against both siphonage and back-pressure, but its broader protection means it's not described as the device primarily for siphonage. A gate valve merely controls flow and does not prevent backflow in the event of pressure changes.