All check valves are essentially used to prevent the reverse flow (or backflow', i.e. in the opposite direction to that intended) of fluids and gases through a pipeline system.

A double check valve is a device designed to help keep your water supply clean and untainted by implementing this very rule, but provides certain key advantages over a single check valve.

How does a double check valve work?

• A double check valve is an in-line device for prevention of backflow, consisting of two single check valves laid out in sequence
• Note that double check valves should not be confused with two-way check valves (such as those used in air braking systems, which work by selecting the higher pressure of two input sources)
• The fact that the double check valve features two valves arranged in sequence means that the device can still operate functionally, even if one check valve somehow becomes jammed open in use
• The closure one valve also lowers the overall the pressure differential across the other
• This in turn means that the double check valve can create a more reliable seal, which helps to prevent even minor leakage back through the system in the wrong direction

Double check valves are very common features of many modern backflow and back siphonage prevention systems, such as those found in everyday domestic/workplace equipment such as combi boilers, fire sprinklers and lawn irrigation networks.

Double check valves are less commonly used as standalone backflow prevention devices in higher-hazard applications, though, where an even more robust type of check valve system is often required in order to comply with industry codes and regulations.