In an RPZ, what maintains the reduced pressure between the checks?

• A. The zone is vented to atmosphere
• B. The zone remains at equal pressure to supply
• C. The zone is pressurized above supply
• D. The zone between the two checks is kept at reduced pressure (at least 2 psi below supply) by the differential pressure relief valve.

Answer: (D)

The key idea is that the space between the two check valves in an RPZ is actively kept at a reduced pressure relative to the supply. This is done by the differential pressure relief valve, which vents that zone to drain as needed to hold it at about 2 psi below the supply. By maintaining this lower pressure, the RPZ remains capable of detecting and preventing backflow, since any tendency for downstream pressure to rise or for backflow to occur will be reflected in the reduced-pressure zone and be vented appropriately. Venting the zone all the time would waste water and isn’t how it operates; keeping equal pressure to the supply would remove protection, and pressurizing the zone above the supply would defeat the backflow prevention purpose.