Backflow: Overview

What is a fluid category?

Schedule 1 of the water fittings regulations in England, Wales and Northern Ireland, byelaws in Scotland, classify the health risk posed by potential contaminants using a scale called fluid categories. 

There are five fluid categories in total the lowest being 1 no risk the highest 5 a serious health hazard. 

Fluid Category 




Wholesome (drinking) water supplied by the undertaker

water direct from a water undertaker’s main


Wholesome (drinking) water which has been changed either heated or altered in taste, odour or appearance  

Hot water 


Fluids posing a slight health hazard

low toxicity chemicals such as common disinfectants


Fluids posing a significant health hazard.

Toxic substances such as pesticides and environmental organisms


Fluids posing a serious health hazard

Pathogenic organisms, radioactive or very toxic substances such as faecal matter



To improve this information please give us your feedback >
Your Feedback


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.