Backflow Prevention

The water supply system must be designed and used in such a way as to prevent contamination from backflow.

Code Requirements

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.

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 public network 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 public network provider and the Building Code, and to protect building users.

Air Gap

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, a dedicated water supply is required 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.

If mains supply is used to top up a private water supply, backflow can be prevented by using a floating to operate a valve, ensuring that the maximum water level always remains at least 25 mm below the mains inlet. Alternatively, a double non-return valve can be used.

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.

Vacuum breaker

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;

  • Atmospheric vacuum breaker (AVB) is one of the simplest and least expensive backflow prevention devices and can provide excellent protection against backsiphonage.
  • Hose connection vacuum breaker (HCVB) is a specialised type of atmospheric vacuum breaker designed to attach directly to the hose tap. It is non-testable and should not be used as protection against backpressure or be subject to continuous pressure (2 hours maximum is permitted), i.e. no control valves should be located downstream of the device.
  • Pressure vacuum breaker (PVB) evolved due to a need to have a vacuum breaker that can be subject to constant pressure and is able to be tested in line. They have two isolating valves and two cocks for testing, one for each chamber. These devices can be used under constant pressure but do not protect against backpressure. They must be at least 300 mm higher than any downstream piping.

Double non-return valve assembly (or double check valve assembly DCVA)

Consists of two independently operated non-return valves within one body. One non-return valve simply acts as a back-up. Because there is a risk that both valves will fail at the same time, regular testing is imperative, and the device is limited to use in medium and low hazard situations. This valve will protect against backpressure and backsiphonage but is not fail-safe. Because of the spring pressures, there can be a significant reduction in pressure (up to 40 kPa) across this device.

Reduced pressure zone Device

This backflow protection device incorporates two independently-acting, spring-loaded check valves separated by a differential pressure relief valve. Pressure between the two valves is lower than the supply pressure during normal operation. If either check valve leaks, the pressure relief valve will open, discharging water out of the system. This device provides the maximum protection of any valve and can be used in high hazard situations.

Backflow prevention device

Typical applications

Air gap

Taps, sinks, tanks

Vacuum breakers (VB)

Industrial plants, cooling towers, laboratories, laundries, hair salons, 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

Installation requirements

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.