What backflow is, how backsiphonage and backpressure cause it, where the path comes from, and why the device must suit the form that can occur.
Backflow is water flowing back towards the supply instead of out to the user. It matters because water that has been in contact with a contaminant can carry it into pipework that other people drink from.
Backsiphonage: pulled back
The supply pressure falls below atmospheric, a negative pressure, and draws water back towards the main, like drinking through a straw. A burst main or heavy demand elsewhere can cause it. It needs a path: an outlet in contact with used water.
- A hose left submerged in a bucket, pond or trough.
- A flexible spray hose long enough to reach below the flood level of a basin, bath or WC pan.
- A tap outlet below the spill-over level of the appliance it fills.
The tap still flows outwards in normal use; the risk appears only when the supply pressure drops.
Backpressure: pushed back
The pressure downstream of a fitting rises above the supply pressure and pushes water back. A booster pump, or thermal expansion in a heated system, can create it. Both forms can occur on hot or cold pipework.
Why the difference matters
Protection must be rated for the fluid category and for every form of backflow that can occur. Most recognised arrangements are rated against both. Some category 5 arrangements, such as an AUK3 tap gap, protect against backsiphonage only, so a downstream pump that can create backpressure rules them out even though the category matches.
At every connection, ask two questions: what could flow back, and could it be pulled back, pushed back or both?