Short definition
Back siphonage is the backflow mechanism that occurs when supply-side pressure drops below atmospheric — a vacuum — pulling contaminated water from a downstream connection back into the potable supply. Common triggers include utility main breaks, fire-flow draw on a nearby hydrant, and water-main shutdowns for repair. Protected against by air gaps, AVBs, PVBs, and RPZ assemblies.
What it is
When supply pressure inverts, any submerged outlet downstream becomes a siphon. The classic example: a garden hose dropped in a swimming pool, a chemical sprayer, or a fertilizer applicator. Under normal conditions, supply pressure pushes water out the hose. When the supply pressure drops below atmospheric — a main break upstream, for example — the hose end becomes the siphon’s mouth, and pool water (or chemical solution) gets drawn back up the hose and into the home’s drinking water.
This is one of the two backflow mechanisms (the other is back-pressure). Back-siphonage is driven by negative supply-side pressure, not by positive downstream pressure. Mechanisms can occur separately or together — a major utility outage during a fire-suppression event, for example.
Protection options:
- Air gap. Geometric — vertical separation between outlet and basin rim. Can never be defeated by pressure inversion.
- AVB (atmospheric vacuum breaker). Mechanical, ASSE 1001, for non-continuous use.
- PVB (pressure-vacuum breaker). Mechanical, ASSE 1020, for continuous-pressure systems like irrigation.
- HBVB (hose-bib vacuum breaker). Mechanical, ASSE 1011, for outdoor sillcocks.
- RPZ. High-hazard assembly that protects against both back-siphonage and back-pressure.
Why it matters to a homeowner
Back-siphonage is the contamination scenario behind several familiar safety rules:
- Hose-end attachments (sprayers, fertilizer applicators) must have integrated check valves or hose-bib vacuum breakers. Submerged hose-ends are the most common back-siphonage exposure in residential settings.
- Hose bibs require integrated or screw-on HBVBs by code.
- Toilet fill valves have built-in vacuum breakers above the overflow tube.
- Air-gapped fixtures (kitchen and bathroom faucets with the spout above the basin rim) are inherently protected and need no additional mechanical device.
A neighborhood-wide back-siphonage event — a major main break upstream — can pull contaminants from any unprotected submerged outlet across many homes simultaneously. The cumulative protection from air-gapped fixtures and code-required vacuum breakers is what keeps the public water system safe during routine utility events.