Page 308 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

 Table 5.8 Air gaps at taps, valves, fittings and cisterns

Situation Nominal size of inlet, tap, valve or Vertical distance between tap or
fitting valve outlet and the spill-over level of
the receiving appliance or cistern
Domestic situation with fluid categories 2 Up to and including G ½ 20 mm
and 3 (AUK2) Over G ½ and up to G ¾ 25 mm

Over G ¾ 70 mm
Non-domestic situation with fluid Any size of inlet pipe Minimum diameter of 20 mm or twice the
categories 4 and 5 (AUK3) diameter of the inlet pipe, whichever is
the greater of the two

A good example of the use of a type AA air gap is in the form of animal drinking
troughs, where the discharge of water into the trough is in a raised housing on
the edge of the trough. The housing is covered to prevent the animals from
having access to the water supply.

Rigidly fixed float- Removable cover Water level controlled
operated valve locked in position to be not less than
25 mm below spill-
p Figure 5.63 Animal trough over level

Type AA air gap

Separation plate prevents damage by animals

p Figure 5.64 Animal trough schematic

Type AB air gap with weir overflow

This gives protection against fluid category 5. It is a non-mechanical backflow
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,
INDUSTRY TIP which discharges into the receptacle, to the critical level of the rectangular weir
overflow.
The size of the weir needs The type AB air gap is suitable for high-risk fluid category 5 situations and is
to be calculated based on
the inlet size. This is usually particularly suited to installations where the contents of the cistern need to be
completed using a weir protected from contaminants such as insects, vermin and dust. A good example
overflow calculator. of this is feed and expansion cisterns in industrial/commercial installations, or
where high-quality water is required, such as in dental surgeries.

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