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Water Supply (Water Fittings) Regulations 1999
Guidance Document relating to
Schedule 1: Fluid Categories and
Schedule 2: Requirements For Water Fittings
[See Regulation 4(3)]

 


 

SECTION 6

Back flow protection: Schedules 1 and 2

SECTION 6.1

Schedule 1: Fluid categories

 

Fluid category 1
Wholesome water supplied by a water undertaker and complying with the requirements of regulations made under section 67 of the Water Industry Act 1991(a).

 

Fluid category 2
Water in fluid category 1 whose aesthetic quality is impaired owing to-

  1. a change in its temperature, or

  2. the presence of substances or organisms causing a change in its taste, odour or appearance, including water in a hot water distribution system.

Fluid category 3
Fluid which represents a slight health hazard because of the concentration of substances of low toxicity, including any fluid which contains-

  1. ethylene glycol, copper sulphate solution or similar chemical additives, or

  2. sodium hypochlorite (chloros and common disinfectants).

 

Fluid category 4
Fluid which represents a significant health hazard because of the concentration of toxic substances, including any fluid which contains-

  1. chemical, carcinogenic substances or pesticides (including insecticides and herbicides), or

  2. environmental organisms of potential health significance.

 

Fluid category 5
Fluid representing a serious health hazard because of the concentration of pathogenic organisms, radioactive or very toxic substances, including any fluid which contains-

  1. faecal material or other human waste;

  2. butchery or other animal waste; or

  3. pathogens from any other source.

 

SECTION 6.2

Schedule 2: Paragraph 15 Back flow prevention

 

  1. (1)   Subject to the following provisions of this paragraph, every water system shall contain an adequate device or devices for preventing back flow of fluid from any appliance, fitting or process from occurring.
     

    (2)   Paragraph (1) does not apply to-

    1. a water heater where the expanded water is permitted to flow back into a supply pipe, or
    2. a vented water storage vessel supplied from a storage cistern, where the temperature of the water in the supply pipe or the cistern does not exceed 25°C.

     

    (3)   The device used to prevent back flow shall be appropriate to the highest applicable fluid category to which the fitting is subject downstream before the next such device.
     

    (4)   Back flow prevention shall be provided on any supply pipe or distributing pipe-
     

    1. where it is necessary to prevent back flow between separately occupied premises, or
    2. where the water undertaker has given notice for the purposes of this Schedule that such prevention is needed for the whole or part of any premises

     

    (5)   A back flow prevention device is adequate for the purposes of paragraph (1) if it is in accordance with a specification approved by the regulator for the purposes of this Schedule.

SECTION 6.3

Regulator's specification for back flow prevention arrangements and devices

 

S15.1 General interpretation of terms relating to back flow prevention
 

"An air gap" means a visible, unobstructed and complete physical air break between the lowest level of water discharge and the level of potentially contaminated fluid downstream (critical water level) within a cistern, vessel, fitting or appliance, hereinafter called a receptacle, that:
 

  1. is not less than 20 mm or twice the internal diameter of the inlet pipe whichever is the greater; and

  2. from which water discharges at not more than 150 from the vertical centreline of the water stream.
     

"Critical level" means the physical or piezometric level of the fluid in any part of the receptacle a minimum of two seconds after closing the water inlet, starting from maximum water level.
 

"Maximum level" means the highest physical or piezometric level of the fluid reached in any part of the receptacle when operated continuously under fault conditions.
 

"Spillover level" means the level at which the or fluid in a receptacle will first spill over the top edge of a receptacle if the inflow of water exceeds the outflow through any outlet and any overflow pipe.
 

"Tap gap" means the vertical distance between the lowest part of a tap outlet and the spillover level of the appliance or receptacle over which the tap discharges.
 

An "upstand" means either one of two alternative arrangements of water fittings to prevent back flow by backsiphonage:
 

Type A upstand

An upward flowing supply or distributing pipe surmounted by an anti-vacuum valve (Type DA), or an anti-vacuum valve combined with a single check valve (Type DUK1), any part of the outlet of which is located not less than 300mm above the spillover level of an appliance.

 

Type B upstand

A branch pipe serving an appliance, where the height of any part of the branch connection to the vented distributing pipe is not less than 300 mm above:

  1. the spillover level of the appliance; or
  2. the highest possible discharge point served by the vented distributing pipe, whichever is the highest.

 

"A verifiable back flow prevention device" means a device, consisting of one or more back flow prevention elements, which can be tested in-situ; usually achieved by the provision of test ports immediately upstream, and between, the mechanical elements comprising the device.

 

S15.2   Interpretations of back flow prevention arrangements as listed in Table S6.1
 

"Type AA - Air gap with unrestricted discharge" means a non-mechanical back flow prevention arrangement of water fittings where water is discharged through an air gap into a receptacle which has at all times an unrestricted spillover to the atmosphere.
 

"Type AB - Air gap with weir overflow" means a non-mechanical back flow prevention arrangement of water fittings complying with Type AA, except that the air gap is the vertical distance from the lowest point of the discharge orifice which discharges into the receptacle, to the critical level of the rectangular weir overflow.
 

"Type AC - Air gap with vented submerged inlet and circular overflow" means a non-mechanical back flow prevention arrangement of water fittings with a vented, but submerged, inlet; the air gap being measured vertically downwards from the lowest point of the air inlet to the critical level.
 

"Type AD - Air gap with injector" means a non-mechanical back flow prevention arrangement of water fittings with a horizontal injector and a physical air gap of 20 millimetres or twice the inlet diameter, whichever is the greater.
 

"Type AF - Air gap with circular overflow" means a non-mechanical back flow prevention arrangement of water fittings with an air gap measured downwards from the lowest point of the discharge orifice, which discharges into the receptacle, to the critical level.
 

"Type AG - Air gap arrangement with minimum size circular overflow" means a non-mechanical back flow prevention arrangement of water fittings with an air gap; together with an overflow, the size of which is determined by measure or a vacuum test.
 

"Type AUK1 - Air gap with interposed cistern" means a non-mechanical back flow prevention arrangement consisting of a cistern with a Type AG overflow and an air gap; the spill-over level of the receiving vessel (WC pan or other receptacle) being located not less than 300 millimetres below the overflow pipe and not less than 15 millimetres below the lowest level of the interposed cistern.
 

"Type AUK2 - Domestic tap gap" means the height of air gap between the lowest part of the outlet of a tap, combination fitting, shower head or other fitting discharging over a domestic sanitary appliance or other receptacle, and the spillover level of that appliance, where a fluid category 2 or 3 risk is present downstream,
 

Type AUK3 - Higher risk tap gap" means the height of an air gap between the lowest part of the outlet of a tap, combination fitting, shower head or other fitting discharging over any appliance or other receptacle, and the spillover level of that appliance, where a fluid category 4 or 5 risk is present downstream,
 

"Type DC - Pipe interrupter with permanent atmospheric vent" means a non-mechanical back flow prevention device with a permanent unrestricted air inlet, the device being installed so that the flow of water is in a vertical downward direction.

 

S15.3   General interpretations of back flow prevention devices as listed in Table S6.2
 

"Type BA - Verifiable back flow preventer with reduced pressure zone" means a verifiable mechanical back flow prevention device consisting of an arrangement of water fittings with three pressure zones with differential obturators and that will operate when potential back flow conditions obtain or there is a malfunction of the valve.
 

"Type CA - Non-verifiable disconnector with different pressure zones" means a non-verifiable mechanical back flow prevention device which provides disconnection by venting the intermediate pressure zone of the device to the atmosphere when the difference of pressure between the intermediate zone and the upstream zone is not greater than 10% of the upstream pressure.
 

"Type DA - Anti-vacuum valve (or vacuum breaker)" means a mechanical back flow prevention device with an air inlet which is closed when water within the device is at or above atmospheric pressure but which opens to admit air if a vacuum occurs at the inlet to the device.
 

"Type DB - Pipe interrupter with atmospheric vent and moving element" means a mechanical back flow prevention device with an air inlet closed by a moving element when the device is in normal use but which opens and admits air if the water pressure upstream of the device falls to atmospheric pressure, the device being installed so that the flow of water is in a vertical, downward direction.
 

"Type DUK1 - Anti-vacuum valve combined with a single check valve" means a mechanical back flow prevention device comprising an anti-vacuum valve with a single check valve located upstream.
 

"Type EA - Verifiable single check valve" means a verifiable mechanical back flow prevention device which will permit water to flow from upstream to downstream but not in the reverse direction.
 

"Type EB - Non-verifiable single check valve" means a non-verifiable mechanical back flow prevention device which will permit water to flow from upstream to downstream but not in the reverse direction.

"Type EC - Verifiable double check valve" means a verifiable mechanical back flow prevention device consisting of two verifiable single check valves in series, which will permit water to flow from upstream to downstream but not in the reverse direction.
 

"Type ED - Non-verifiable double check valve" means a non-verifiable mechanical back flow prevention device consisting of two single check valves in series, which will permit water to flow from upstream to downstream but not in the reverse direction.
 

"Type HA - Hose union back flow preventer" means a mechanical prevention back flow device for fitting to the outlet of a hose union tap and consisting of a single check valve with air inlets that open if the flow of water ceases.
 

"Type HC - Diverter with automatic return" means a mechanical back flow prevention device used in bath/shower combination tap assemblies which automatically returns the bath outlet open to atmosphere if a vacuum occurs at the inlet to the device.
 

"Type HUK1 - Hose union tap incorporating a double check valve" means a hose union tap in which a double check valve has been incorporated into either the inlet or outlet of the tap.
 

"Type L - Pressurised air inlet valve" means an anti-vacuum valve or vacuum breaker, similar to Type DA but suitable for conditions where the water pressure at the outlet of the device under normal conditions of use is greater than atmospheric.
 

"Type LB - Pressurised air inlet valve combined with a check valve downstream" means a mechanical back flow prevention device comprising a Type LA anti-vacuum valve and a single check valve located downstream.
 

Table S6.1: Schedule of non-mechanical back flow prevention arrangements and the maximum permissible fluid category for which they are acceptable

Type

Description of back flow prevention arrangements and devices

Suitable for protec-tion against 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 (For example, a WC suite)

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
Vertical distance of bottom of tap outlet above spill-over level of receiving appliance
Not exceeding G ½
Exceeding G ½ but not exceeding G ¾
Exceeding G ¾
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:

a   any domestic or non-domestic sink or other appliance; or

b  any appliances in premises where a higher level of protection is required, such as some appliances in hospitals or other health care premises,

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:
1 X Indicates that the back flow prevention arrangement or device is not applicable or not acceptable for protection against backpressure for any fluid category within water installations in the UK.

2 Arrangements incorporating Type DC devices shall have no control valves on the outlet of the device; they shall be fitted not less than 300 mm above the spillover level of a WC pan, or 150 mm above the sparge pipe outlet of a urinal, and discharge vertically downwards .

3 Overflows and warning pipes shall discharge through, or terminate with, an air gap, the dimension of which should satisfy a Type AA air gap.

 



Table S6.2: Schedule of mechnical back flow prevention arrangements and the maximum permissible fluid category for which they are acceptable

Type

Description of back flow prevention arrangements and devices

Suitable for protection against fluid category

Back-pressure

Back-siphonage

BA

Verifiable back flow 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 back flow preventer. Only permitted for use on existing hose union taps in house installations

2

3

HC

Diverter with automatic return (Normally integral with some domestic appliance applications only)

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:
1 X Indicates that the back flow prevention device is not acceptable for protection against backpressure for any fluid category within water installations in the UK.

2 Arrangements incorporating a Type DB device shall have no control valves on the outlet of the device. The device shall be fitted not less than 300mm above the spillover level o f an appliance and discharge vertically downwards .

3 Types DA and DUK1 shall have no control valves on the outlet of the device and be fitted on a 300 mm minimum Type A upstand.

4 Relief outlet ports from Types BA and CA back flow prevention devices shall terminate with an air gap, the dimension of which should satisfy a Type AA air gap.


SECTION 6.4

Guidance clauses relating to Schedule 1: Fluid categories; and Paragraph 15 of Schedule 2: Back flow prevention

 

Guidance

 

General

G15.1   Except where expanded water from hot water systems or instantaneous water heaters is permitted to flow back into a supply or distributing pipe, every water fitting through which water is supplied for domestic purposes should be installed in such a manner that no back flow of fluid from any appliance, fitting or process can take place.
 

G15.2   Avoidance of back flow should be achieved by good system design and the provision of suitable back flow prevention arrangements and devices, the type of which depends on the fluid category to which the wholesome water is discharged. A description of fluid risk categories is shown in Schedule 1 of the Regulations and some suggested examples relating to the fluid categories are shown in Tables 6.1a to e.
 

G15.3   The type of back flow protection for a given situation is related to the fluid risk categories downstream of the back flow prevention device.
 

G15.4   Schedules of back flow prevention arrangements and back flow prevention devices, and the maximum permissible fluid risk category for which they are acceptable, are shown in Table S6.1and Table S6.2. Details of the arrangements and devices are shown in Table 6.2and Table 6.3.
 

G15.5   Wherever practicable, systems should be protected against back flow without the necessity to rely on mechanical back flow protection devices; this can often be achieved by point of use protection such as a 'tap gap' above the spillover level of an appliance. Minimum air gaps for different sizes of taps and applications are shown in Table S6.1.
 

G15.6   In cistern fed systems secondary back flow prevention can often be achieved for appliances by the use of permanently vented distributing pipes. See Figure 6.2b.
 

G15.7   Mechanical back flow protection devices which, depending on the type of device, may be suitable for protection against backpressure or backsiphonage, or both, should be installed so that:

 

  1. they are readily accessible for inspection, operational maintenance and renewal; and,
  2. except for Types HA and HUK1, back flow prevention devices for protection against fluid categories 2 and 3, they should not be located outside premises; and,
  3. they are not buried in the ground; and,
  4. vented or verifiable devices, or devices with relief outlets, are not installed in chambers below ground level or where liable to flooding; and,
  5. line strainers are provided immediately upstream of all back flow prevention devices required for fluid category 4. Where strainers are provided, servicing valves are to be fitted upstream of the line strainer and downstream of the back flow prevention device; and,
  6. the lowest point of the relief outlet from any reduced pressure zone valve assembly or similar device should terminate with a Type AA air gap located not be less than 300mm above the ground or floor level; and,
     

Note: For information on the installation and maintenance of reduced pressure zone devices (RPZ valve assemblies) see Installation and Guidance Note No. 9-03-02 published by the Water Regulations Advisory Scheme]

 

Appliances incorporating, or supplied with water through, pumps

G15.8   Where pumped showers, or other appliances supplied through or incorporating pumps, are installed, care should be taken in positioning branches from distributing pipes.

 

Bidets (including WCs adapted as bidets) with flexible hose and spray handset fittings and with submerged water inlets

G15.9   Bidets with flexible hose and spray handset fittings and/or water inlets below the spillover level of the appliance, are a fluid category 5 risk and should not be supplied with water directly from a supply pipe.

G15.10   Bidets of this type may:

  1. be supplied with cold and/or hot water through Type AA, AB, or AD back flow prevention arrangements serving the bidet only; or,
  2. be supplied with cold water from an independent distributing pipe serving the bidet only, see Figure 6.1a; or a common distributing pipe serving the bidet and which may also serve a WC or urinal flushing cistern only; or,
  3. be supplied with hot water from a water heater, which is supplied from an independent distributing pipe, that serves the bidet only, see Figure 6.1a; or,
  4. where the bidet is at a lower elevation than any other outlets or appliances, be supplied with water from a common cold and/or hot water vented distributing pipe providing that:
    1. the elevation of the spillover level of the bidet, if there is no flexible hose; or,
    2. the elevation of the spray outlet, with the hose extended vertically above the spillover level of the bidet,
       

    whichever is the highest, is not less than 300 mm below the point of connection of the branch pipe serving the bidet to the main distributing pipe serving other appliances.
     

Bidets with water inlets above spillover level only
 

G15.11   Bidets in domestic locations with taps or mixers located above the spillover level of the appliance, and not incorporating an ascending spray inlet below spillover level or spray and flexible hose, may be served from either a supply pipe or a distributing pipe provided that the water outlets discharge with a Type AUK2 air gap above the spillover level of the appliance. See Table S6.1.

 

WCs and urinals
 

G15.12   The water supply to a manually operated WC or urinal flushing valve may be derived either from a supply pipe or a distributing pipe. The flushing valve should be located above the WC pan or urinal and must incorporate, or discharge through, a pipe interrupter with a permanent atmospheric vent; see Type DC in Table S6.1and Table 6.2. The lowest part of the vent opening of the pipe interrupter should be located not less than 300 mm above the spillover level of the WC pan or not less than 150 mm above the sparge outlet of a urinal. See Figure 6.1b for typical installation details.

 

Shower heads or tap inlets to baths, washbasins, sinks and bidets
 

G15.13    Except where suitable additional back flow prot