During a backflow condition, water could be discharged from a relief port on the valve. Although in the majority of such occurrences they don't fully discharge, a significant volume of water can flow when they do. If not carefully designed to provide for such an incident, discharge from a backflow device could cause major damage to a building and its contents.
Under certain circumstances, the system pressure, or pressure downstream from the backflow device can exceed the upstream water pressure. Similarly, the pressure upstream from a backflow device could drop, whereby the downstream pressure is higher. In either of these conditions, the system water attempts to migrate back through the valve. Instead of allowing this potentially contaminated water to do so, the valve relief port opens and spills the water. It continues to spill water until the pressure imbalance is corrected.
The volume of water discharged from the relief port is a function of the zone pressure and valve size. Since flow can be large (Fig. 1), the best location for a backflow device is usually outside the building. In colder climates; however, precautions must be taken to properly protect the backflow device from freezing temperatures.
In many cases, indoor installation is necessary. When installing backflow devices indoors, suitably sized drains must be available. Many existing installations incorporate a relief valve port-sized air gap fitting with a port-sized discharge to a floor sink or floor drain. Should a major backflow condition occur, few of these systems will be capable of handling the resultant flow.
To ensure proper drainage, the manufacturer of each valve should be consulted to determine the maximum flow rate from the relief port. The air gap, drain line and drainage receptor should be sized accordingly. Backflow preventer manufacturers offer an auxiliary alarm kit, which provides an alarm in the event the relief port on the valve opens. However, until the valve is physically turned off, major water damage could result. To avoid potential problems, size the relief line correctly (see Fig. 2).