Which backflow prevention device maintains a reduced pressure between the supply and the protected system to prevent backflow?

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A reduced pressure zone device is designed to keep a lower pressure in the space between its two checks and the protected system, so water cannot flow backward into the supply. It uses a relief valve that vents to drain whenever the pressure in that zone tries to rise toward the supply or if an upstream drop in supply occurs. This maintained reduced-pressure zone is what stops backflow, even in high-hazard situations, because any tendency for reverse flow causes the relief valve to open and release water rather than letting it reverse into the clean supply.

An air gap accomplishes backflow prevention by physically separating the supply outlet from the receives; there’s no pressure relationship to manage, just a safe distance that prevents siphoning. A double check valve relies on two checks to block reverse flow but does not create or vent a reduced-pressure zone. And “backflow preventer” is a general term that can refer to different devices, not specifically the reduced-pressure-zone arrangement described here.