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A cross connection is any point where potable water could be contaminated by a non-potable source. An air gap prevents backflow physically; mechanical devices do it with valves and require testing.
The plumbing topic with the most serious consequences, which is why it appears more than its share.
What a cross connection is
Any actual or potential connection between the potable water supply and a source of contamination.
A hose lying in a bucket of chemicals is a cross connection. So is a boiler feed without protection, or an irrigation system tied into the domestic supply.
Two ways it flows backwards
| Mechanism | Cause |
|---|---|
| Backsiphonage | Negative pressure in the supply pulls water back |
| Backpressure | Pressure downstream exceeds supply pressure and pushes water back |
A water main break causes backsiphonage. A pump or a boiler causes backpressure. Different mechanisms, different devices.
The air gap
A physical separation between a supply outlet and the flood rim of a fixture. Water cannot flow up through air, so it cannot backflow at all.
Every device has valves that can stick, foul or wear. An air gap is geometry. That is why it is treated as the highest form of protection and why questions frame it that way.
Devices
Reduced pressure zone assemblies, double check valve assemblies, pressure vacuum breakers and atmospheric vacuum breakers, each suited to a different hazard level and pressure condition.
Testable devices need periodic testing by a certified tester. That obligation is ongoing and it is commonly forgotten after handover.
Common questions
What is a cross connection?
Any actual or potential connection between the potable supply and a source of contamination.
What are the two backflow mechanisms?
Backsiphonage from negative supply pressure, and backpressure from downstream pressure exceeding supply pressure.
What is an air gap?
A physical separation between a supply outlet and the flood rim of a fixture. Water cannot flow up through air.
Why is an air gap the best protection?
It cannot fail mechanically. Every device has valves that can stick or wear.
Do backflow devices need testing?
Testable assemblies require periodic testing by a certified tester, and that obligation continues after handover.