Backflow Prevention in Residential Plumbing

What it is

Backflow is the reverse flow of water from a private plumbing system back into the drinking water supply. It can carry chemicals, sewage or other contaminants into the main and put public health at risk. Backflow prevention is the set of devices and installation rules that stop this from happening.

In Australia the requirements come from AS/NZS 3500.1 Water services, called up by NCC Volume Three. Every connection to a water main needs to be assessed for backflow hazard and fitted with a containment device that matches the assessed risk. Ongoing testing of testable devices is governed by AS/NZS 2845.3.

Why backflow happens

There are two physical causes. Back-siphonage happens when pressure in the main drops below pressure in the private system, usually because of a main break or heavy fire-fighting draw. Back-pressure happens when a pressurised system on the property (a boiler, a booster pump, an elevated tank) pushes water back against the supply pressure.

A residential property without backflow protection can contaminate the street main with garden chemicals, swimming pool water, recycled water or rainwater tank water. The device sits between the private system and the main so anything that flows backwards is stopped or contained.

Hazard ratings under AS/NZS 3500.1

AS/NZS 3500.1 defines three hazard ratings that determine which device is acceptable at the property boundary.

High hazard

Any condition, device or practice connected to a drinking water system that could cause death. Examples include sewage handling, chemical dosing, mortuaries and some industrial processes. In residential settings, high hazard typically applies to properties with recycled water (purple pipe) connections or large in-ground irrigation systems with chemical injection.

Medium hazard

Any condition that could endanger health but not cause death. Examples in residential settings include in-ground swimming pools, rainwater tank cross-connections to mains, and properties with significant garden irrigation.

Low hazard

Conditions that constitute a nuisance but do not endanger health. Standard residential properties without special hazards typically fall into this category.

Device selection by hazard

The device installed at the property boundary must match the assessed hazard rating.

High hazard devices

• Registered air gap (RAG)
• Registered break tank (RBT)
• Reduced pressure zone device (RPZD)

These are all testable and require annual testing.

Medium hazard devices

• Double check valve assembly (DCVA), testable
• Pressure vacuum breaker assembly, testable

A non-testable dual check is not acceptable for medium hazard.

Low hazard devices

• Dual check valve (non-testable)
• Single check valve where permitted by the water authority

Most residential homes without special hazards use a non-testable dual check at the meter. Many water authorities now require a dual check at every new residential connection by default.

Annual testing requirement

AS/NZS 3500.1 requires that testable backflow devices be commissioned at initial installation. AS/NZS 2845.3 governs ongoing field testing and maintenance, with testing at intervals not exceeding 12 months. The test must be performed by a licensed plumber who holds the backflow prevention specialist class endorsement.

The test result is lodged with the water authority that owns the main. If the device fails, it must be repaired or replaced and retested. Continued failure to lodge an annual test can result in the water authority disconnecting the supply.

Non-testable dual checks do not require annual testing but should be inspected during any plumbing work on the property and replaced if found to be faulty.

Where devices go

The containment device sits as close as practical to the property boundary on the downstream side of the water meter. The point is to protect the main from anything that happens inside the property.

Zone protection and individual fixture protection may also be required inside the property in addition to containment at the boundary. For example, an irrigation system with chemical injection needs an RPZD on the irrigation feed even if the boundary already has a DCVA. Hose taps connected to garden irrigation may need a hose connection vacuum breaker. The boundary device alone does not always cover every internal hazard.

Records and accreditation

The plumber who installs or tests a backflow device must be licensed by the relevant state authority and hold the backflow specialist class. In Victoria this is the VBA, in NSW it is NSW Fair Trading, and in Queensland it is the QBCC. Each test result and each device installation must be recorded and lodged with the water authority. Records are typically kept for the life of the device.

Failure to install a required device, or to maintain annual testing on a testable device, is a plumbing offence and can result in fines and disconnection of supply.