Why are there 2 backflow devices on this 8” firemain?

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If you've ever looked at an 8-inch fire main and wondered why it has two backflow preventers, the answer often comes down to the fire flow meter and the requirements for accurate testing and system protection.

A fire flow meter introduces pressure loss (head loss) as water passes through it, especially during high-flow fire testing. On a large 8-inch fire main, a single backflow preventer combined with a flow meter may not provide enough capacity to deliver the required fire flow without creating excessive pressure drop. By installing two backflow devices in parallel, the available flow capacity is increased while reducing friction loss through each assembly.

During normal operation, one assembly may handle the domestic or low-flow demand, while during fire events or flow testing, both assemblies can pass the large volume of water needed. This helps maintain the required residual pressure for the fire sprinkler or hydrant system while still protecting the public water supply from backflow contamination.

Although adding a second backflow preventer increases installation and testing costs, it is often the most practical solution when a flow meter is required on a high-capacity fire service. The design allows the system to meet fire protection demands, satisfy local water utility requirements, and maintain reliable backflow protection without sacrificing performance when every gallon per minute counts.