Page 567 - 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 9 Sanitation systems

The ventilated branch discharge system is used on larger systems where there is
a risk of trap seal loss because the waste pipe lengths are excessive. Control of
the pressure in the waste pipe (the discharge branch) is achieved by ventilating
it no further than 750 mm from the appliance as this safeguards against trap
seal loss by induced or self-siphonage. Alternatively, small air admittance valves
may be used at each appliance. These allow air into the system when the
appliance is in operation.

Secondary ventilated stack system

Secondary ventilating stack

Rodding eye

Branch discharge pipe

Main discharge stack

p Figure 9.8 Secondary ventilated stack system

With a secondary ventilated stack system, only the main discharge stack is
ventilated. This system arrangement safeguards against positive and negative
pressure fluctuations.

The rules regarding branch ventilating pipes
Where branch ventilating pipes must be installed, the following rules apply.
● Any branch ventilating pipe must be connected to the discharge stack
above the spill-over level of the highest appliance fitted to the stack. The
ventilating pipe must also rise away from the appliance.
● The minimum size of any ventilating pipe to a single appliance is 25 mm.
However, if it is longer than 15 m, or the ventilating pipe serves more than
one appliance, then the size must be 32 mm.
● The main ventilation stack must be a minimum of 75 mm. This also applies
to the dry part of the primary ventilating stack.

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