Page 297 - 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

Air gap 2 inlet diameter Not less than 25 mm
Not less than 25 mm
Service Overflow
valve
pipe

Shut off
level
Warning alarm
Alarm sounds when the water is
25 mm from the invert of the
overflow pipe
Cistern capacity greater than 5000 L
Type 'AG' and 'AF' air gaps
p Figure 5.50 Cistern with a capacity greater than 5000 litres

In both cases, the size of the overflow pipe will depend upon the type of air gap
incorporated into the cistern (we will look at air gaps and backflow protection
a little later in this unit) and this will depend upon the fluid category of the
cistern contents. It must be remembered that:

● if a type AG air gap (fluid category 3) is fitted, the overflow diameter shall be
a minimum of twice the inlet diameter
● if a type AF air gap (fluid category 4) is fitted, the minimum cross-sectional
area of the overflow pipe must be, throughout its entire length, four times
the cross-sectional area of the inlet pipe
● for all cisterns greater than 1000 litres, the invert of the overflow must not
be less than 50 mm above the working level of the cistern.
Multiple cistern installations:

interconnection of two or more cisterns

Where large quantities of water are required but space is limited, cisterns can be
interlinked, provided the cisterns are of the same size and capacity. Problems can
occur if the cisterns are not linked correctly, especially where the cisterns are to
supply drinking water. Stagnation of the water in some parts of the cistern may
cause the quality of the water to deteriorate. It should be remembered that the
number of cisterns to be linked should be kept to a minimum.
Stagnation can be avoided by following some basic rules. Connection must
be arranged to encourage the flow of water through each cistern. This can be
achieved by:

● keeping the cistern volumes to a minimum to ensure rapid turnover of water
and thus prevent stagnation
● connecting the cisterns in parallel wherever possible
● connecting the inlets and the outlets at opposite ends of the cistern
● using delayed-action float-operated valves to limit stratification.

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