Which backflow prevention device would normally be used on the feed water line directly to a low-pressure boiler that uses chemical treatment of the feed water?

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

Answer: (B)

When you have a boiler feed line that uses chemical treatment, the risk is backflow carrying those chemicals into the potable water supply. To protect against both back-siphonage and backpressure in this high-hazard situation, a reduced pressure principle backflow preventer is used. It has two check valves with a relief valve between them that vents to drain if pressure on the supply side drops or if backpressure from the boiler side occurs. This creates an isolated, monitored barrier so contaminated boiler water can’t reverse into the public supply, which is why it’s the appropriate device for chemical-treated boiler feed.

A double check valve provides basic backflow protection but is not considered adequate for lines that involve hazardous chemicals or continuous pressure from a boiler, so it isn’t the preferred choice here. An atmospheric vacuum breaker relies on atmospheric venting and isn’t suitable for pressurized feed lines or scenarios with chemical additives, since it can’t protect against backpressure. An isolation valve isn’t a backflow preventer at all; it only allows you to shut off flow and does not prevent cross-connection contamination.