For a feed water line to a boiler with chemical treatment, which backflow prevention device is typically used?

• A. Atmospheric vacuum breaker
• B. Reduced pressure principle valve
• C. Double check valve
• D. Isolation valve

Answer: (B)

Preventing backflow from a line that carries chemical-treated boiler feed water requires a device that protects against both backsiphonage and backpressure in a high-hazard cross-connection. The standard choice for this scenario is a reduced pressure principle backflow preventer. It uses two check valves with a vented relief chamber that maintains a reduced pressure between the supply and the protected system. If backflow starts to occur, the relief valve opens to drain, preventing contaminated boiler water or chemicals from reaching the potable supply. This design provides robust protection even if one check valve leaks or pressures fluctuate, which is essential when dangerous chemicals are involved.

An atmospheric vacuum breaker isn’t suitable here because it relies on atmospheric venting and isn’t designed for continuous pressure or high-hazard cross-connections, making it unreliable for boiler feed lines. A simple double check valve offers some protection but doesn’t provide the vented relief and is generally not rated for high-hazard cross-connections like chemical feeds to boilers. An isolation valve isn’t a backflow prevention device at all; it only shuts off flow and does not prevent backflow.