Backflow is the unwanted reverse flow of water or other substances into drinking-water pipes. It is prevented by removing cross-connections or protecting them with a suitable air gap or backflow prevention device. An appropriately qualified plumber should determine which backflow preventer and installation point are required, then handle the installation and testing.
How backflow happens
Backflow requires a cross-connection, which is an actual or potential connection between drinking water and a non-potable source. Reverse flow can then occur in two ways:
- Back-siphonage: Supply pressure falls below the pressure at a connected outlet. For example, a pressure drop in the water main could draw liquid through a submerged garden hose.
- Back pressure: Pressure within a property exceeds the incoming supply pressure. A pump connected to a tank or process system could push non-potable water towards drinking-water pipes.
Common cross-connections include hoses submerged in pools, tanks, buckets or chemical solutions. Keep hoses above the highest possible liquid level and disconnect them when not in use.
Unusual colour, odour or pressure does not confirm backflow. If contamination is suspected, stop using the affected water for drinking, cooking or brushing teeth, and contact your water provider and a licensed plumber. Do not taste the water or attempt to dismantle plumbing devices.
How to prevent backflow
Eliminate the cross-connection where possible. If a connection must remain, protection may include:
- An air gap: Unobstructed vertical separation between a water outlet and the highest possible liquid level.
- A backflow prevention device: Depending on the risk and pressure conditions, this may use check valves, vacuum-breaking components or a pressure-relief arrangement.
Vacuum breakers, double-check valve assemblies and reduced-pressure-zone devices are not interchangeable. Selection depends on the contamination hazard, installation layout and whether back-siphonage, back pressure or both are possible.
Not every house needs the same device or location. Protection may be required at an individual outlet, within the property’s plumbing or at the property boundary. A plumber should assess the hazard and confirm the current Sydney Water, plumbing code and applicable standards requirements.
Do not install, bypass, repair or test a backflow device yourself. In NSW, regulated plumbing work requires an appropriately licensed professional. Check who can install or test a backflow preventer, as some testable devices require periodic licensed backflow testing and reporting under the relevant water provider’s current requirements.
Drinking-water devices versus sewer valves
A sewer backflow valve, also called a backwater or reflux valve, restricts wastewater flowing backwards through sanitary drainage. It does not protect drinking-water pipes and cannot replace a potable-water device. The methods used to prevent sewer backflow address a different risk.
If sewage is backing up, avoid contact, keep people and pets away, and call a licensed plumber. Do not open or dismantle the valve. For a property-specific drinking-water assessment, contact our backflow prevention plumbers.