Concepts
Backflow Fundamentals
What backflow is, its two causes, and why cross-connection control protects public health.
What Is Backflow
BACKFLOW is the undesirable REVERSE flow of water (or other liquids/gases) into the potable water supply. Water is supposed to flow one way — from the supply to the user. Backflow reverses that, potentially pulling contaminants into the clean water everyone drinks.
A CROSS-CONNECTION is any actual or potential connection between the potable supply and a source of contamination. Backflow prevention is fundamentally about controlling cross-connections.
The Two Causes
Backflow has exactly two causes: • BACK-SIPHONAGE — caused by NEGATIVE pressure (a partial vacuum) in the supply. Like sipping through a straw, low/negative supply pressure siphons contaminated water back. Caused by water main breaks, firefighting demand, or undersized piping. • BACK-PRESSURE — caused when the downstream (user) pressure is GREATER than the supply pressure, pushing water backward. Caused by pumps, boilers, elevated tanks, or thermal expansion.
Degree of Hazard
Hazards are classified by severity: • HIGH HAZARD (contaminant) — could cause illness or death (sewage, chemicals, irrigation with chemicals). • LOW HAZARD (pollutant) — objectionable but not a health threat (e.g. aesthetic issues).
The degree of hazard determines which backflow assembly is required — high hazard needs the highest level of protection.
📖 Key Terms
- Backflow
- The undesirable reverse flow of water into the potable supply.
- Cross-connection
- Any actual or potential link between potable water and a contamination source.
- Back-siphonage
- Backflow caused by negative (sub-atmospheric) supply pressure.
- Back-pressure
- Backflow caused by downstream pressure exceeding supply pressure.
💡 Exam Tips
- ▸The two causes of backflow are back-siphonage and back-pressure.
- ▸Back-siphonage = negative supply pressure; back-pressure = higher downstream pressure.
- ▸A cross-connection is any potential link between potable water and contamination.
- ▸Degree of hazard (high vs. low) determines the required assembly.