Page 569 - 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

Air admittance valves
An air admittance valve allows air into a stub stack to prevent the loss of trap
seals. The subsequent suction action, when an appliance is used, opens the
valve. This stabilises the air condition in the stack because air is sucked into the
stack through the valve, also preventing smells and foul air escaping out. When
the appliance has finished its operation, the valve closes, preventing smells
escaping into the space where the valve is installed.

Air admittance valves should be fitted in a non-inhabited space such as a roof
space. This minimises the risk of freezing while keeping the valve accessible.
On no account should they be fitted outside because of the risk of the valves
freezing up in the closed position during cold weather. If air admittance valves p Figure 9.11 The operation of
are installed within a boxing, the boxing must be ventilated. In all cases, the an air admittance valve
valve must be accessible for repair or replacement.

The requirements are that one stack in five must be ventilated to the outside air KEY POINT
using a conventional ventilation stack, and that this should usually be done at An important point to
the head or start of the drain run. The general rules are as follows: remember is that air
admittance valves are
● Up to four domestic properties of no more than three storeys high can be not a substitute for
ventilated using air admittance valves. ventilation stacks and
● Where an underground drain serves more than four properties fitted with an any drain where an air
air admittance valve, the following rules apply. admittance valve is
● Where five to ten buildings exist, additional conventional ventilation fitted will still require
stacks must be installed at the head of the drain run. conventional venting
● Where 11 to 20 buildings exist, additional conventional ventilation stacks at some point. This is
simply to minimise the
must be installed at the head of the drain and at the mid-point in the run effects of back pressure,
of the drain. which could occur if the
● All multi-storey domestic properties will require additional conventional underground drainage
ventilation if more than one property is fitted with an air admittance system becomes
valve and is connected to a common drain that is not ventilated by a blocked.
conventional ventilation stack.
Connecting multiple waste appliances to branch
discharge pipework

The connection of two or more appliances on a single waste pipe is often
installed incorrectly on the primary ventilated stack. This is usually the cause of
baths pulling the water from the trap of a wash basin.

Where multiple appliances are to be installed, then the use of ventilating branch
pipework should be considered to avoid trap seal loss (see the section on the
ventilated branch discharge system, page 554).

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