A backflow prevention arrangement or device is in effect a barrier intended to prevent contaminated fluid flowing backwards.
Backflow protection can be provided by an air or tap gap arrangement or a mechanical backflow prevention device approved be the Regulator (Regulators’ Specification), or alternatively authorised as a relaxation. (Relaxations allow a water undertaker the discretion to accept an arrangement as preventing backflow.)
The Regulators’ Specification not only provides definitions but also 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.
As some backflow prevention arrangements and devices have operational limitations you should always check with the local water undertaker to make sure they are suitable for the intended application. Wherever practicable plumbing systems should be protected against backflow without the necessity to rely on mechanical backflow protection devices.
Please note: if high risk contaminants are likely to be present in a plumbing system, because these represent a greater risk to health, in addition to any point of use protection the water undertaker can also require the installation of zone or wholesite backflow protection.
Regulators Specification: Non-mechanical backflow protection | |||
Type | Description | Suitability by fluid category | |
Back pressure | Back siphonage | ||
AA | Air gap with unrestricted discharge above spillover level | 5 | 5 |
AB | Air gap with weir overflow | 5 | 5 |
AC | Air gap with vented submerged inlet | 3 | 3 |
AD | Air gap with injector | 5 | 5 |
AF | Air gap with circular overflow | 4 | 4 |
AG | Air gap with minimum size circular overflow determined by measure or vacuum test | 3 | 3 |
AUK1 | Air gap with interposed cistern | 3 | 5 |
AUK2 | Air gaps for taps and combination fittings (tap gaps) discharging over domestic sanitary appliances, such as a washbasin, bidet, bath or shower tray shall not be less than the following: Size of tap or combination fitting . Not exceeding G ½ Exceeding G½ but not exceeding G ¾ Exceeding G ¾
Vertical distance bottom of tap outlet above spillover level of recieving appliance 20 mm 25 mm 70 mm
| X | 3 |
AUK3 | Air gaps for taps or combination fittings (tap gaps) discharging over any higher risk domestic sanitary appliances where a fluid category 4 or 5 is present, such as:
Shall be not less than 20 mm or twice the diameter of the inlet pipe to the fitting, whichever is the greater | X | 5 |
DC | Pipe interrupter with permanent atmospheric vent | X | 5 |
Notes:
| |||
Regulators Specification: Mechanical backflow protection | |||
Type | Description | Suitability by fluid category | |
Back pressure | Back siphonage | ||
BA | Verifiable backflow preventer with reduced pressure zone | 4 | 4 |
CA | Non-verifiable disconnector with difference between pressure zones not greater than 10% | 3 | 3 |
DA | Anti-vacuum valve (or vacuum breaker) | X | 3 |
DB | Pipe interrupter with atmospheric vent and moving element | X | 4 |
DUK1 | Anti-vacuum valve combined with a single check valve | 2 | 3 |
EA | Verifiable single check valve | 2 | 2 |
EB | Non-verifiable single check valve | 2 | 2 |
EC | Verifiable double check valve | 3 | 3 |
ED | Non-verifiable double check valve | 3 | 3 |
HA | Hose union backflow preventer. Only permitted for use on existing hose union taps in house installations | 2 | 3 |
HC | Divertor with automatic return (normally integral with some domestic appliance applications). | X | 3 |
HUK1 | Hose union tap which incorporates a double check valve. Only permitted for replacement of existing hose union taps in house installations | 3 | 3 |
LA | Pressurised air inlet valve | X | 2 |
LB | Pressurised air inlet valve combined with a check valve downstream | 2 | 3 |
Notes:
| |||
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 AC air gap is a non-mechanical backflow prevention arrangement comprising of a vented, but submerged inlet that discharges into a cistern, vessel, fitting or appliance fitted (receiving vessel) with an outlet and circular overflow.
A Type AC air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure for contamination risks no greater than fluid category 3 .
A summary of some of the key requirements applicable to a Type AC air gap is given below:
The air gap is an unobstructed and complete physical break between the lowest point of the vent (air inlet) and the critical water level of the weir overflow. It must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.
The critical level 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.
Supply pipes maybe submerged but adjustable or dismantlable joints on the are not permitted below the critical water level.
The overflow shall be not less than 19mm (internal diameter).
Where they discharge to drain the overflow and warning pipe must be fitted with an air break to drain or equivalent prior to the drain connection.
Type AC air gap installations should be inspected, and as necessary maintained every 12 months (BS EN 806: 5)
A Type AD air gap, sometime call a ‘jump jet’ is a non-mechanical backflow prevention arrangement comprising of a horizontal injector which ‘jets’ water into a cistern, vessel, fitting or appliance fitted (receiving vessel) and an air break to drain open to atmosphere.
A Type AD 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 AD air gap is given below:
The air gap is an unobstructed and complete physical break between the terminal point of the injector and inlet orifice of the receiving vessel. Measured horizontally it must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.
The air break to drain must be open to atmosphere and capable of draining the maximum flow rate without submerging or coming into contact with the inlet.
The inlet must not come into contact with any splashing, drips or run off from the receiving vessel.
Type AD air gap installations should be inspected, and as necessary maintained every 6 months (BS EN 806: 5)
A Type AF air gap is a non-mechanical backflow prevention arrangement comprising of an inlet which discharges water into a cistern, vessel, fitting or appliance fitted (receiving vessel) fitted with an outlet and circular overflow.
A Type AF air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure for contamination risks no greater than fluid category 4.
A summary of some of the key requirements applicable to a Type AF air gap is given below:
The air gap is an unobstructed and complete physical break measured downwards between the lowest point of discharge from the inlet and the critical water level. It must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.
The critical level 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
The overflow must be circular and of a minimum size throughout its length (four times in the inlet pipe cross sectional area). It must be capable of draining the maximum inflow of water under fault conditions i.e. when the outlet is closed but the inlet continues to discharge.
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.
Where they discharge to drain the overflow and warning pipe must be fitted with an air break to drain or equivalent prior to the drain connection.
Type AF air gap installations should be inspected, and as necessary maintained every 12 months (BS EN 806: 5)
A Type AG air gap is a non-mechanical backflow prevention arrangement comprising of an inlet which discharges water into a cistern, vessel, fitting or appliance fitted (receiving vessel) fitted with an outlet and minimum sized circular overflow.
A Type AG air gap is rated by the Regulators as suitable backflow protection against both back siphonage and back pressure for contamination risks no greater than fluid category 3.
A summary of some of the key requirements applicable to a Type AG air gap is given below:
The air gap is an unobstructed and complete physical break measured downwards between the lowest point of discharge from the inlet and the critical water level. It must be no less than 20 mm or twice the internal diameter of the supply whichever is the greater.
The critical level 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
The overflow must be circular and a minimum of 19mm throughout its length. It must be capable of draining the maximum inflow of water under fault conditions i.e. when the outlet is closed but the inlet continues to discharge.
Supply pipes maybe submerged but adjustable or dismantlable joints on the are not permitted below the critical water level. If 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.
Where they discharge to drain the overflow and warning pipe must be fitted with an air break to drain or equivalent prior to the drain connection.
Type AG air gap installations should be inspected, and as necessary maintained every 12 months (BS EN 806: 5)
A Type AUK1 air gap is a non-mechanical backflow prevention arrangement comprising of an interposed cistern fed via an inlet arrangement forming a Type AG air gap, and an outlet arrangement which discharges the contents by gravity into a separate receiving vessel. The receiving vessel (for example a WC pan or cistern) must be located a minimum distance below the interpose cistern. Depending on the outcome of an assessment by the local water undertaker can feed a single or multiple installations.
A Type AUK 1 air gap is rated by the Regulators as suitable backflow protection against back pressure risks no greater fluid category 3 and back siphonage risks up to and including fluid category 5.
A summary of some but not all of the requirements applicable to a Type AUK1 air gap is given below:
The interposed cistern must conform to the requirements for a Type AG air gap.
The maximum fluid level in the receiving vessel must be a minimum of 300 mm below the spillover level of the interposed cistern and invert (lowest point of flow which may be different to the lowest point of discharge) of the warning pipe.
The maximum fluid level in the receiving vessel must be a minimum of 15 mm below the lowest internal water level of the interposed cistern.
The fluid in the interposed cistern must be categorised as no greater than fluid category 3.
As it includes a Type AG air gap, Type AUK 1 installations should be inspected, and as necessary maintained every 12 months (BS EN 806: 5)
Type AUK 2 tap gap is suitable for low risk installations (no greater than fluid category 3). 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 low risk receptable it supplies.
A Type AUK 2 tap gap is rated by the Regulators as suitable backflow protection against contamination risks no greater than fluid category 3 but for back siphonage only.
The requirements for a Type AUK2 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 will vary depending upon the connection size of the tap or combination fitting as given below:
Connection size
Not exceeding ½“
Exceeding ½”but not exceeding ¾”
Exceeding ¾”
Tap gap
≥ 20 mm
≥ 25 mm
≥ 70 mm
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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