Back siphonage is a system condition that can occur if there is a sudden interruption of a supply to a property, thus creating a vacuum in the pipework system, which can cause backflow.
Backpressure is a system condition created when the incoming pressure in a supply is less than the pressure within the plumbing system, which can also cause backflow.
Backflow means flow in a direction contrary to the intended normal direction of flow, within or from a water fitting.
Fluid Categories and Risks, as Scheduled in the Water Supply (Water Fittings) Regulations 1999
Fluid Category 1: Wholesome water supplied by the water undertaker and complying with the requirements of regulations made under section 67 of the Water Industry Act 1991 (a), i.e. water supplied directly from the water undertaker’s main.
Fluid Category 2: Water in fluid category 1 whose aesthetic quality is impaired as a result of:
(a) a change in its temperature, or
(b) the presence of substances or organisms causing a change
in its taste, odour or appearance, including water in a hot
water distribution system.
Examples of causes of such impairment are:
Mixing of hot and cold water supplies.
Domestic softening plant (common salt regeneration).
Drink vending machines in which no ingredients or carbon dioxide are injected into the supply or distributing inlet pipe.
Fire sprinkler systems (without anti-freeze).
Ice making machines.
Water cooled air conditioning units (without additives).
Fluid Category 3: Fluid which represents a slight health hazard because of the concentration of substances of low toxicity, including any fluid which contains:
(a) ethylene glycol, copper sulphate solution, or similar
chemical additives; or
(b) sodium hypochlorite (chloros and common disinfectants).
Examples of such fluids are those contained in:
Primary circuits and heating systems
(with or without additives) in a house.
Domestic washbasins, baths and showers.
Domestic clothes and dishwashing machines.
Home dialysing machines.
Drink vending machines in which ingredients or carbon dioxide are injected.
Commercial softening plant (common salt regeneration only)
Domestic hand-held hoses with flow-controlled spray or shut-off control,
Hand held fertiliser sprays for use in domestic gardens.
Domestic or commercial irrigation systems, without insecticide or fertiliser additives, and with fixed sprinkler heads less than 150mm above ground level.
Fluid Category 4: Fluid which represents a significant health hazard due to the concentration of toxic substances, including any fluid which contains:
(a) chemical, carcinogenic substances or pesticides (including
insecticides and herbicides); or
(b) environmental organisms of potential health significance.
Examples of such fluids are those contained in:
General
Primary circuits and central heating systems in non-domestic environments. Fire sprinkler systems using anti-freeze solutions.
House Gardens
Mini irrigation systems without fertilizer or insecticide application: such as pop-up sprinklers or permeable hoses.
Food Processing
Food preparation, dairies and bottle washing apparatus.
Catering
Commercial dishwashing machines, bottle washing apparatus and refrigerating equipment.
Industrial and Commercial Applications
Dyeing equipment
Industrial disinfection equipment.
Printing and photographic equipment.
Car washing and degreasing plants.
Commercial clothes washing plants.
Brewery and distillation plant.
Water treatment plant or softeners using other than salt.
Pressurised fire-fighting systems.
Fluid Category 5: Fluid which represents a serious health hazard because of the concentration of pathogenic organisms, radioactive or very toxic substances, including any fluid which contains:
(a) Faecal material or other human waste; or
(b) Butchery or other animal waste; or
(c) Pathogens from any source.
Examples of such fluids are those contained in:
General
Industrial cisterns, non-domestic hose union taps, sinks, urinals, WC pans and bidets, Permeable pipes in other than domestic gardens, laid below or at ground level, with or without permeable additives, and grey water re-cycling schemes.
Medical
Any medical or dental equipment with submerged inlets, laboratory equipment, bedpan washers, mortuary and embalming equipment, hospital dialysing machines. Commercial clothes washing plant in healthcare premises. Non-domestic sinks, baths, washbasins and other applications.
Food Processing
Butchery and meat trades, slaughterhouse equipment, vegetable washing.
Catering
Dishwashing machines in healthcare premises. Vegetable washing.
Industrial and Commercial Applications
Industrial and chemical plant etc., mobile plant, tankers and gully emptiers. Laboratories. Sewage treatment and sewer cleansing. Drain cleaning plant. Water storage for agricultural purposes. Water storage for fire-fighting purposes.
Commercial Agricultural
Commercial irrigation outlets below or at ground level and/or permeable pipes, with or without chemical additives. Insecticide or fertilizer applications. Commercial hydroponic systems. The foregoing are examples of various fluid categories but are not exhaustive.
| 
