Unit 307 Session 10 Fluid categories

​What is backflow/back siphonage?

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

​Backflow can also happen within your home, under certain circumstances dirty water from your bath or washbasin could find itself running back through your drinking water tap.

​B​ackflow/back siphonage protection in bathrooms

All water fittings in a bathroom must be supplied via an appropriate and adequate form of backflow protection which is rated either equal to or higher than the highest downstream contamination risk (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. 

​Below are some examples of where there may be a risk in a bathroom.

​T​oilets

A toilet is considered to be a fluid category 5 risk.

Typically, toilets have built in fluid category 5 protection such as a Type AUK1 backflow arrangement

​Bath and basin taps

The backflow risks associated with bath and basin taps will vary dependent upon their use (intended or otherwise) and the environment in which they are sited.
There are a number of ways in which to prevent backflow via a tap, one of the most commonly used forms being a tap gap.

A tap gap is a ‘gap’ between the lowest point of the tap outlet and spill over level of the bath, basin or bidet, the size of which varies depending upon the diameter of the pipe supplying the tap.

​Mixer and bi-flow taps

​Single and double check valves

​High Risk Taps
Taps used in high risk installations, such as kitchen sinks, laboratories and medical facilities are considered to be a fluid category 5 risk.

Backflow protection is usually provided in the form of an AUK3 tap gap i.e. an air gap of 20 mm or twice the internal diameter of the inlet pipe, whichever is the greater.

​Hose union taps
Hose union taps are typically categorised as a fluid category 3 or 5 risk.