Water is meant to flow one direction — from the network, into the tank, out to use. Backflow is what happens when that direction reverses, and a cross-connection between a storage tank and any other system is exactly the kind of path that makes reversal possible.
Backpressure
Backpressure occurs when downstream pressure — from a pump, an elevated system, or a thermal expansion effect — exceeds the supply pressure, pushing water (and anything mixed into it) backward through a connection into the potable network rather than forward as intended.
Back-siphonage
A sudden drop in supply pressure — a main break, a large fire flow draw, or a supply interruption — can create a vacuum effect that draws water backward from a connected system into the network, potentially carrying contamination with it if that connected system was not itself potable-grade or was cross-connected to a non-potable source.
Auxiliary or secondary water sources
A site with a bore, recycled water, or rainwater system connected — even indirectly, even with the intention of never mixing them — alongside a potable network creates a cross-connection risk unless a properly designed and tested backflow prevention device separates the two.
Fire system connections
Fire systems are a particularly important cross-connection point because they are less frequently used than the general potable supply, meaning stagnant water and any backflow event can go unnoticed for longer. A dedicated backflow prevention device on the fire system connection is standard practice for exactly this reason.
A cross-connection is not automatically a contamination event — it is a pathway that makes one possible under the right (or wrong) pressure conditions. Backflow prevention devices exist to close that pathway, not to fix a contamination event after it has already occurred.
| Element | Why it matters |
|---|---|
| Identification of all cross-connection points | You cannot protect a connection you have not identified |
| Backflow prevention device sized and rated to the hazard level | Different connection risks require different device types |
| Regular testing of installed backflow devices | A device can fail silently without periodic verification |
| Records of testing and device certification | Supports compliance and provides evidence during audit or incident review |
| Review after any change to connected systems | A new connection changes the cross-connection risk profile |
Does every water storage tank need a backflow prevention device?
Not automatically — the need depends on whether the tank has any actual or potential cross-connection to another system. A tank supplied and drawn from exclusively within a single, uncontaminated potable network has a lower backflow risk than one connected to multiple sources or systems.
How often should backflow prevention devices be tested?
Testing frequency depends on the applicable regulatory requirement and the hazard rating of the specific connection, but periodic testing — commonly annual for many device types — is standard practice, since a device can fail without any visible external sign.
Is a fire system connection really a significant backflow risk if it is rarely used?
Yes — infrequent use is part of why fire system connections are a notable risk, since stagnant conditions and any backflow event are less likely to be quickly noticed compared to a heavily used connection. This is why fire system cross-connections are commonly required to have dedicated backflow protection.
Not certain your site's cross-connections are properly protected against backflow? A review can identify the gaps before they become a contamination event.
Request a backflow risk review