Backflow Prevention: What It Is and Why It Matters

Backflow prevention doesn’t come up until something else forces the question — installing a JoJo tank, connecting a borehole, adding irrigation, or a compliance certificate flags it during a property sale. Here’s what it actually is, why it matters, and when it’s genuinely required.

What backflow is, and why it happens

Municipal water pipes are supposed to keep water flowing one way — from the supply into your home. Backflow is when that direction reverses, and water (or whatever’s mixed into it on your side of the connection) gets pulled back into the municipal line instead. It’s not a hypothetical: a sudden drop in municipal pressure — a water outage, a nearby main break, or a large draw during firefighting — can create suction that pulls water backward through any connection that isn’t protected against it.

The risk isn’t the water itself, it’s what might be in it on your side of the pipe by the time it gets pulled back: stored tank water, garden chemicals from an irrigation system, pool water treated with chlorine and other chemicals, or anything a borehole pump has drawn in. Once that’s been pulled into the municipal line, it doesn’t just affect your property — it’s now in the shared supply.

Where it actually matters in a home

Not every tap needs a backflow preventer. The risk is specifically wherever your plumbing connects the municipal supply to a secondary water source or an outlet that isn’t just a plain tap:

• JoJo tanks and backup water systems — anywhere a tank is plumbed in with a changeover valve so the house can draw from either supply
• Borehole connections — if borehole water is ever blended into or plumbed alongside the municipal line
• Irrigation and garden hose systems — a hose left submerged in a bucket of fertiliser or pesticide, or an irrigation system with chemical injection, is a classic backflow scenario
• Pool auto-fill lines — pool water is chemically treated and shouldn’t have a path back into the drinking supply
• Commercial and industrial connections — process water, cooling systems, or anything with additives on the property side

A standalone tap with nothing plumbed into it isn’t a meaningful backflow risk on its own. It’s specifically the point where two systems connect that needs protection.

Types of backflow prevention devices

The right device depends on the level of risk at that connection point:

• Vacuum breakers — the simplest option, suited to lower-risk connections like an irrigation system or an outdoor tap
• Double check valve assemblies — two check valves in series, a common choice for JoJo tank and backup water system connections
• Reduced pressure zone (RPZ) devices — the highest level of protection, generally required for higher-risk connections like borehole blending or commercial/industrial process water

Sizing and choosing between these isn’t something to guess at — it depends on the specific connection, the water pressure involved, and what’s on the other side of it.

When it’s actually required for compliance

Backflow prevention isn’t just good practice — it’s frequently a specific requirement for a PIRB compliance certificate wherever a secondary water source is plumbed into the same system as the municipal supply. If you’re installing a JoJo tank with a changeover valve, connecting a borehole, or adding an irrigation system with any kind of chemical feed, this is usually the point a compliance certificate will flag if it’s missing. It matters most at exactly the moments people are least likely to think about it — mid-installation of something else, when the backflow device feels like an afterthought rather than the point of the job.

Testing and maintenance

Backflow preventers aren’t a fit-and-forget device, particularly RPZ assemblies — the internal check valves and relief mechanism can wear or stick over time, and a device that’s failed silently gives no warning until it’s actually needed. Where a higher-risk device is installed, periodic testing is worth budgeting for as part of ongoing maintenance, not treated as a one-time installation cost.

What a proper installation should include

The correct device type for the specific connection (not a generic one-size choice), correct sizing for the pressure and flow involved, and installation at the right point in the system — as close to the connection as practical, not tacked on somewhere convenient. If a JoJo tank, borehole, or irrigation quote doesn’t mention backflow prevention at all, it’s worth asking why before the work starts, not after a compliance certificate comes back flagged.