What is backflow and back pressure?

Backflow is where the water flows back towards the supply source, either by siphonic action (back siphonage) or forced back under pressure (back pressure). This could be from taps, hoses or other water carrying equipment. Where water comes into contact with materials, chemicals or other fluids, these too could be ‘pulled back’ with the water and potentially contaminate your drinking water system.

Whatever the potential source of contamination, the water we supply to our customers must be protected. For this reason the Regulations require adequate protection to be built in and/or installed to prevent backflow from occurring. The Regulations place potential contaminants and substances into one of five fluid categories dependant on the harm it may cause. It also specifies the types of devices for preventing backflow that may be used and the fluid categories they are able to protect against.

How do I know what protection device I need?

When you understand what your risks are, it is easy to work out what level of protection you need.

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Fluid category Description Example Prevention


Wholesome water supplied by your water company United Utilities None needed


Water with a slight change of temperature, taste or odour
  • Ice machine
  • Fire sprinkler systems (without antifreeze)
  • Water cooled air conditioning united (without additives)
Single Check Valve (Type EC)


Fluid which represents a slight health hazard
  • Drink vending machines in which ingredients or carbon dioxide are injected
  • Washbasins, baths, showers
  • Commercial water softeners
Double Check Valve


Fluid which represents a significant health hazard
  • Dairies
  • Food preparation
  • Brewery and distillation plant
  • Commercial boiler top-up system
RPZ Valve (Type BA device)


Fluid which represents a serious health hazard
  • Gray water recycling systems
  • Medical or dental equipment
  • Vegetable washing
  • Sewage treatment, including sewer and drain cleaning
  • Water for agricultural use
  • Commercial washing machines
AA or AB Air Gap