Backflow Protection: FAQs

What is backflow?

Backflow occurs when fluid flows in the opposite to the intended or normal direction of flow. There are two types of backflow, back pressure and back siphonage.


  • Backpressure: occurs when the pressure downstream increases above that of the supply pressure, in effect pushing fluids backwards against the intended or normal direction of flow.


  • Back siphonage: occurs when the supply pressure drops below that of the system it is supplying creating a depressurisation or vacuum which pulls fluids backwards against the intended or normal direction of flow.


The type of backflow risk is dependent upon a number of factors. Good design and the installation of suitable backflow prevention arrangements are key to avoiding backflow, which is why notification is so important.

As the circumstances which could lead to backflow are a common occurrence across the UK, contamination of public water supplies by backflow of fluids from customers premises is not theoretical. It is an ever present threat to water quality and public health.

As there have been a number of serious contamination events across the UK water undertakers take their role in preventing such incidents very seriously. Where infringements are suspected or identified they will act, not only to protect water quality and public health, but also to support owners and occupiers of premises to meet their legal obligations.

Where contamination incidents occur, it is likely that enforcement action, including criminal proceedings, will be taken against those who fail to meet their legal obligations.



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Uncontrolled if downloaded. This is informative, non-statutory guidance and intended for general guidance purposes only; it is subject to change.

Compliance with this information should not be relied upon as guaranteeing no enforcement action will be taken by water undertakers. Water Regs UK accepts no liability for loss, indirect or consequential loss arising from or in connection with this guidance document.

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