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Nabic 256A Pipe Interrupter Nabic 255 Anti-Pollution Valve
Nabic 256A Pipe Interrupter (DC Type) Nabic 255 Anti-Pollution Valve (RPZ)

Anti-Pollution / Backflow Prevention

A backflow device is used to protect potable water (drinking water) supplies from contamination or pollution due to backflow.

Examples of how a backflow can occur

These are just a few examples but failures like these could poison the main water line in your area.

(a) If the end of a garden hose was submersed in a bucket of chemical whilst it was being filled and there was a loss of water pressure then a backflow could occur.

(b) If a shower hose was submersed in the toilet and there was a loss of water pressure then a backflow could occur.

Backflow Prevention in a bathroom


(c) If a component failed in a boiler, heat exchanger or pumped water distribution system then reverse flow could occur.


Backflow - Categories of Risk

The water supply regulations specify there must be adequate devices at outlets to prevent backflow. The risk from backflow is assessed in terms of fluid categories.

Category 1: No risk. Potable water

Category 2: Water that has undergone a change in taste, odour appearance or temperature. For example water that is heated for bathing, or is discharged from showers, or water that has been treated with a water softener using a salt regeneration process. These changes tend to be aesthetic only and therefore present no threat to human health.

Examples: Mixing of hot and cold water through appliances and taps. Supply of water through a vending machine where no ingredients are added direct into the water supply such as carbon dioxide. Domestic salt regeneration water softeners. Ice making machines Fire sprinkler systems, which do not contain additives. Water-cooled air conditioning units, which do not contain additives.

Category 3: Water that is a hazard to health and is not suitable for drinking, cooking or other domestic purposes. For example water that has been used for bathing.

Examples: Water present in central heating systems. Water used by dishwashers, washing machines, home dialysing machines. Water that has been used for bathing or showering. Garden hoses that have a flow control or shut off control. Garden hoses that are attached to fertilizer sprays. Domestic or commercial irrigation systems, without additives, sprinkler heads to be not less than 150mm above ground level. Vending machines in which ingredients are injected. Commercial salt regeneration water softeners.

Category 4: Water that is a significant hazard to health due to the concentration of toxic substances and is not suitable for drinking, cooking or other domestic purposes.

Examples: Commercial central heating systems. Fire sprinklers that include anti-freeze solutions. Mini irrigation systems that use pop up sprinklers or permeable hoses. Food preparation areas, dairies bottle washing etc. Commercial dishwashers. Car washing and degreasing plants. Industrial disinfection equipment.

Category 5: Water that is a serious hazard to health due to the concentration of pathogenic organisms or toxic substances.

Examples: Pathogens from any source. Human or animal waste. Grey water recycling systems. Medical waste e.g. bedpan washers or commercial laundry plants. Permeable pipes in other than domestic gardens. Drain cleaning plant.


Types of backflow prevention

The simplest most effective way to provide backflow prevention is to provide an air gap but this is not always possible in every application.

Examples of the use of an air gap are:

  • Taps above washbasins
  • Cold water cisterns, where the float valve outlet must be above the overflow water level.
  • Hand-held showers must have their hoses fastened such that the shower head cannot rest below the water level in a bath or basin (see example above).

If an air gap is not possible then a specialised backflow preventer valve may be installed at strategic locations in the system wherever there is a risk of contaminated fluids entering the water supply.

Zoned Backflow Prevention

Backflow Prevention devices

There are various types available but the most common are;

Double check valve (DCVA) (i.e. Nabic 256A)
This is basically two check valves connected in series. In normal operation both valves will provide a very tight shut off against backflow. An additional benefit is that if one valve fails then it will still function. It is commonly used on lawn irrigation and combi-boiler systems but will also have other applications.

Reduced pressure zone device (RPZ) (i.e. Nabic 255)
A recent introduction to the UK has been the Reduced Pressure Zone (RPZ) valve, a form of double check valve where the intervening zone is drained and normally kept empty.  If the downstream valve leaks and permits backflow, this will drain out through the vent rather than building up pressure against the upstream valve. These valves are complex, requiring certified installation and annual checks. They are used for category 4 systems, such as fire sprinklers where the system has an anti-freeze additive.


Safety Valves, Relief Valves, Reducing Valves - Flowstar