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The water supply system must be designed and used to prevent contamination from backflow.
On this page:
- Causes of backflow
- Code requirements
- Using an air gap to prevent backflow
- Backflow prevention devices
- Installation requirements
Backflow is the unplanned reversal of flow of water (or water and contaminants) into the water supply system.
For backflow to occur, there must be a physical connection, or cross-connection, between the water supply and any delivered water or contaminant. A common situation is the end of a garden hose submerged in a bucket or other container of liquid. Backflow can also arise from appliances, pools, and water storage tanks such as header tanks and rainwater tanks.
As well as using the methods described below to minimise the risk of backflow, advise building owners to take simple precautions such as not submerging garden hoses or spray heads from showers and sinks, and always turning off the water supply at the tap when it is not being used.
Causes of backflow
Backflow is caused by a difference in pressure and may occur due to:
- backsiphonage – the supply pressure is less than the downstream pressure, allowing water to be pushed in the wrong direction
- backpressure – for example, insufficient relief of pressure in a vessel where water is heated.
Backflow can only occur where there is a connection or cross-connection. Cross-connections can occur in any situation where fixtures are connected directly to the main supply such as:
- irrigation systems
- washing machines
- coffee machines
- swimming pools, spa pools or ornamental pools that are filled by hose
- water softeners
- pesticide and fertiliser attachments for hoses
- fridges and icemakers
- retractable spray outlets to tubs and sink
- flexible shower hoses
- storage tanks.
Building Code Clause G12 Water supplies requires that potable water supply must be protected from contamination and installed in a manner that avoids the likelihood of contamination within the system.
Acceptable Solution G12/AS1 requires backflow prevention to be provided where it is possible for water or contaminants to backflow into a piped potable water supply. Backflow can be prevented either through an air gap or a backflow prevention device (see below).
The Acceptable Solution also provides that there must be no likelihood of a cross-connection between a private water supply (such as a rainwater tank) and mains water supply.
Responsibility for preventing backflow may rest with:
- the network utility provider who may install a backflow prevention device as part of the meter assembly, or
- the individual property owner whose responsibility it is to comply with the requirements of the network utility provider and the Building Code, and to protect building users.
Using an air gap to prevent backflow
In most situations, an air gap is the most cost-effective and reliable form of backflow prevention.
An air gap should be used to prevent backflow from rainwater tanks and other water supply tanks into the mains-supplied water system. Air gaps should also be used to prevent backflow of contaminants from all appliances and fixtures that are connected to the water supply.
For swimming and spa pools, provide a dedicated water supply with an approved air gap.
Acceptable Solution G12/AS1 requires that the air gap must be the greater of 25 mm, or twice the diameter of the supply pipe.
Backflow prevention devices
If the system is a high pressure system and a pipe is directly connected to an appliance or sanitary fixture, it may not be possible to use an air gap. In this case, a backflow prevention device must be installed.
The appropriate device for a particular installation will depend on the:
- hazard level of any potential contaminant
- potential for cross-connection
- type of backflow expected
- physical limitation of the device and the environment.
Cross-connections are rated according to Building Code Acceptable Solution G12/AS1 to three hazard levels:
- High – this has the potential to cause death
- Medium – this would endanger health
- Low – this is a nuisance but does not endanger health
Generally, the higher the hazard, the higher the risk, so the safer the device must be.
A vacuum breaker contains a float disc and an air inlet port. Under normal water flow, the float disc closes off the air inlet port, but if the normal water flow is interrupted, the float drops, closing off the system against backflow and, at the same time, opening the air inlet port.
A variety of vacuum breakers are available:
Double non-return valve assembly
Reduced pressure zone device
Backflow prevention device
Vacuum breakers (VB)
Industrial plants, cooling towers, laboratories, laundries, swimming pools, lawn sprinkler systems, fire sprinkler systems
Double check valve assembly (DCVA)
In-house pumps, elevated tanks, non-toxic boilers
Reduced pressure backflow assembly (RPBA)
Industrial plants, hospitals, morgues, chemical plants, irrigation systems, pumps, elevated tanks, boilers, fire sprinkler systems
All backflow prevention devices require a building consent for installation and must be:
- installed by a registered plumber
- installed as near as practicable to the potential point of contamination
- protected from physical and frost damage
- isolated from corrosive or toxic environments
- installed above surrounding ground level so that leakage from air ports and discharge ports is readily visible
- installed in a position and manner to be accessible for maintenance and testing
- fitted with a line strainer upstream to prevent particles in the pipework from rendering the device ineffective
- attached only after the pipework has been flushed
- installed without the application of heat.
Backflow prevention devices may be testable or non-testable. Their use in a particular situation depends on the degree of hazard. Non-testable devices may only be used on low-hazard rated systems.
Testable devices must be tested on installation and at regular intervals to the standard set down by Acceptable Solution G12/AS1: 3.7 Testing. Non-testable devices should be checked every 2 years maximum.
Updated: 12 September 2016