Hoses attached to hose union taps are a high contamination risk. Backflow via a hosepipe submerged in a bucket, trough, puddle, drain or pond is a very real possibility. Where fluid category 5 protection is required this is typically provided by installing a Type DC arrangement, break tank suppled via a Type AA or AB air gap, or an arrangement which maintains a Type AUK3 tap gap at all times.
A Type AUK3 is a minimum gap of twice the internal diameter of the supply pipework or 20 mm, whichever is the greater, between the hose outlet and ground or spill over level of whatever it is discharging into. One way to maintain this gap is to restrain the hose so it cannot be lowered compromising the gap. Installing an arm which can swing out will provide a greater reach.
Backflow occurs when fluid flows in the opposite to the intended or normal direction of flow. There are two types of backflow, back pressure and back siphonage.
Backpressure: occurs when the pressure downstream increases above that of the supply pressure, in effect pushing fluids backwards against 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.
The type of backflow risk is dependent upon a number of factors. Good design and the installation of suitable backflow prevention arrangements are key to avoiding backflow, which is why notification is so important.
As the circumstances which could lead to backflow are a common occurrence across the UK, contamination of public water supplies by backflow of fluids from customers premises is not theoretical. It is an ever present threat to water quality and public health.
As there have been a number of serious contamination events across the UK water undertakers take their role in preventing such incidents very seriously. Where infringements are suspected or identified they will act, not only to protect water quality and public health, but also to support owners and occupiers of premises to meet their legal obligations.
Where contamination incidents occur, it is likely that enforcement action, including criminal proceedings, will be taken against those who fail to meet their legal obligations.
Water undertakers are legally required to ensure the drinking water they supply is wholesome. Contamination occurs when there is a change in water quality irrespective of whether or not it is harmful to health
As part of their statutory duty to enforce the water fittings regulations in England, Wales and Northern Ireland, byelaws in Scotland, the local water undertaker will identify the level of backflow protection needed. This categorisation will be based on a number of factors including the highest downstream fluid category risk the fitting is or is likely to be subject.
It is in effect a barrier intended to prevent contaminated fluid flowing backwards.
Schedule 2 paragraph 15 of the water fittings regulations in England, Wales and Northern Ireland, byelaws in Scotland require every plumbing system to incorporate protection against backflow. This is often referred to as point of use backflow protection. It can be provided by an air or tap gap arrangement or a mechanical backflow prevention device.
Backflow prevention arrangements and devices permitted under the regulations/byelaws need to be approved by the regulator or alternatively authorised as a relaxation. Relaxations allow a water undertaker the discretion to accept an arrangement as preventing backflow.
The regulator rates a backflow protection arrangement or device for suitability against contamination risk (fluid categories) and types of backflow risk - back siphonage or back pressure. They should always be accessible for inspection, maintenance and replacement.
Backflow prevention devices, are required to be corrosion resistant. For example, for metallic backflow prevention devices manufactured of gunmetal or other dezincification resistant materials. Dezincification water fittings are typically marked with a CR symbol.
Wherever practicable plumbing systems should be protected against backflow without the necessity to rely on mechanical backflow protection devices.
A Type AA air gap is a non-mechanical backflow prevention arrangement comprising of an inlet which discharges water into a cistern, vessel, fitting or appliance (receiving vessel) and an outlet. Depending on the outcome of an assessment by the local water undertaker it can feed a single or multiple installations.
A Type AA air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure at the highest level of contamination risk, fluid category 5.
A summary of some of the key requirements for a Type AA air gap is given below:
The supply pipe and inlet control must be external to the receiving vessel and fixed so the air gap is maintained and unrestricted.
The air gap is an unobstructed and complete physical break between the lowest point of discharge and the spillover level of the contents of the receiving vessel. Measured vertically it must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.
The spillover level is the level at which the contents of receiving vessel spill over the top edge when the inflow of water exceeds the outflow through the outlet i.e. demand.
The spillover is unrestricted.
If the supply pipe feeding the inlet or the inlet itself comes into contact with the contents of the receiving vessel, for example due to splashing or foaming, then the air gap is considered to be compromised and must be increased to the point no contact occurs.
Type AA air gaps should be inspected, and as necessary, maintained every 6 months (BS EN 806: 5)
A Type AB air gap is a non-mechanical backflow prevention arrangement comprising of an inlet which discharges water into a cistern, vessel, fitting or appliance (receiving vessel) fitted with an outlet and a rectangular weir or ‘slot’ overflow. Depending on the outcome of an assessment by the local water undertaker it can feed a single or multiple installations.
A Type AB air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure at the highest level of contamination risk, fluid category 5.
A summary of some of the key requirements applicable to a Type AB air gap is given below:
The air gap is an unobstructed and complete physical break between the lowest point of discharge and the critical water level of the weir overflow. Measured vertically it must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.
The critical level (sometimes referred to as h) is the fluid level in the receiving vessel under fault conditions i.e. when the outlet is closed but the inlet continues to discharge. It is measured at least 2 seconds after closing the water inlet
Whether sited internally or externally (as shown in the diagram opposite) the weir overflow must be rectangular (non-circular) and capable of accommodating discharge under fault conditions. Where a screened mesh is installed consideration should be given to the impact this may have on discharge flow. The air gap can be confirmed by test or calculation using the Type AB air gap calculator.
Neither the fluid pathway to the overflow nor the discharge from it should be restricted. For example, there should be a sufficient gap between the overflow and any surface to accommodate full discharge unimpeded during fault conditions.
Submerged supply pipes are not permitted. If the supply pipe feeding the inlet or the inlet itself comes into contact with the contents of the receiving vessel, for example due to splashing or foaming, then the air gap is considered to be compromised and must be increased to the point no contact occurs.
Type AB air gap installations, including screened mesh, should be inspected, cleaned and as necessary maintained every 6 months (BS EN 806: 5)
A Type AUK 3 tap gap is suitable for high risk installations (fluid category 5 or lower). It is a specific minimum distance between the lowest point of a tap spout, shower head or other fitting and the spillover level of the high risk appliance or receptable it supplies.
A Type AUK 3 tap gap is rated by the Regulators as suitable backflow protection against the highest level of contamination, fluid category 5 but for back siphonage only.
The requirements for a Type AUK3 tap gap are :
The tap gap is measured vertically from the lowest point of the outlet, which includes any aerator or flow straightener fitted, and the spillover level of whatever receptacle it supplies.
The gap required must be not less than 20 mm or twice the diameter of the inlet pipe supplying the installation whichever is the greater.
Where an AUK3 tap gap cannot be maintained, for example where the tap gap is compromised as a result of:-
the tap being lowered in any way;
adjustment to the spout which reduces the distance between the tap outlet and spillover level of the sink; or
the operation of a pull out hose attachment
An alternative form of fluid category 5 backflow protection is required.
Regulations & Byelaws which apply include but are not limited to:
Relevant guidance includes but is not limited to:
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