Page 307 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 5 Cold water systems

Electric shower

Because the pressure in the main is zero,
gravity forces water in the system back
towards the water main

Water from the bath being
sucked out by back siphonage
towards the water main

Up stream Down stream

Sudden loss of pressure due to a burst on the undertaker's main
p Figure 5.61 Back siphonage

Type AA air gap with unrestricted discharge above
spill-over level
This gives protection against fluid category 5 and is a non-mechanical backflow
prevention arrangement of water fittings, where water is discharged through
an air gap into a cistern, which has, at all times, an unrestricted spill-over
to the atmosphere. The air gap is measured vertically downwards from the
lowest point of the inlet discharge orifice to the spill-over level. It should be
remembered that:
● the type AA air gap is suitable for all fluid categories
● the size of the air gap is subject to the size of the inlet (see Table 5.8)
● the flow from the inlet into the cistern must not be more than 15° from the
vertical.
Air gap

Spill-over level

TYPE AA air gap with unrestricted
discharge above spill-over level
Suitable for protection against fluid category:
Back Back Outlet
5 pressure 5 siphonage

p Figure 5.62 Type AA air gap
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