Backflow
The definition of backflow is the undesirable reversal of flow in a piping system, so that water or other substances move backward toward the potable supply instead of out to fixtures. Backflow has two causes: backsiphonage, when supply pressure drops below the downstream pressure, and backpressure, when downstream pressure rises above the supply. It is prevented with an air gap or a backflow prevention assembly at each cross-connection.
Updated 2026 · Researched from published standards, not professional or legal advice.
Backflow at a glance
| Question | Answer |
|---|---|
| Meaning | Water flowing the wrong way, back toward the drinking water supply |
| Where it happens | At a cross-connection: any link between potable water and a possible contaminant |
| Cause 1: backsiphonage | Supply pressure falls (main break, hydrant flow, pump failure) and siphons liquid backward |
| Cause 2: backpressure | Downstream pressure rises above supply pressure (pumps, boilers, elevation) |
| How it is prevented | An air gap, or a backflow prevention assembly (RP, double check, PVB, SVB) matched to the hazard |
| Who verifies the protection | A certified backflow tester, on the cycle the water purveyor sets |
Backflow is the reversal of the normal direction of flow in a water system. Instead of clean water traveling from the public main out to fixtures and equipment, water or other liquids move backward through the piping toward the source. Backflow happens through two distinct mechanisms: backsiphonage, which is driven by a drop in supply pressure, and backpressure, which occurs when a downstream system pushes harder than the supply. Both mechanisms require a cross-connection to actually contaminate anything.
The reason backflow receives so much regulatory attention is that a single reversal event can contaminate far more than the building where it starts. Once a contaminant enters the service line it can spread into the distribution system and reach other customers before anyone notices a change in taste, color, or odor. Many contaminants are invisible in water, so the danger is not reliably self-announcing. Preventing the reversal in the first place is far more dependable than detecting it afterward.
For testers and property owners, the practical takeaway is that backflow prevention is about controlling conditions, not predicting them. Pressure loss from a main break or heavy firefighting draw, or pressure gain from a pump or heated vessel, can occur without warning at any hour. Protection therefore relies on devices and assemblies that work automatically, and on periodic testing to confirm that those mechanical safeguards still close tightly and operate as designed.
Common questions
What does backflow mean in plumbing?
In plumbing, backflow means water in a building or irrigation system flowing backward into the drinking water piping, and from there possibly into the public main. Plumbing codes require every cross-connection to be protected because any reversal can carry whatever was in the downstream system with it.
What is backsiphonage?
Backsiphonage is backflow caused by negative or reduced pressure in the supply: a water main break, heavy fire hydrant use or a failed booster pump drops the supply pressure, and the downstream system siphons backward like a straw. Vacuum breakers are designed for this case.
What is a backflow preventer?
A device or assembly installed at a cross-connection to stop reversed flow. Testable assemblies, such as the reduced pressure principle assembly and the double check valve assembly, have test cocks and shutoff valves so a certified tester can verify each check annually.
Sources
- U.S. EPA, Cross-Connection Control Manual (Office of Water): definitions of backflow, backsiphonage, backpressure and the assembly types.
- University of Southern California Foundation for Cross-Connection Control and Hydraulic Research, Manual of Cross-Connection Control: field test procedures for RP, double check and vacuum breaker assemblies.
Related terms
| Backsiphonage | Backflow caused by a drop or loss of pressure in the supply piping that siphons water backward through a cross-connection. |
| Backpressure | Backflow caused when pressure downstream of the potable connection becomes greater than the pressure of the supply itself. |
| Cross-connection | Any actual or potential physical link between a potable water system and a source of contamination or pollution. |
| Backflow incident | An event in which contaminated or non-potable water actually enters a potable water system through a cross-connection. |
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