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TECHNICAL
34
T H E C A N A D I A N
HOME INSPECTOR
V
enting is a very important part of waste
plumbing. It allows waste and water
to flow properly. The venting performs
three functions. It allows air in front of
the water flowing through the waste pipe to be
pushed out of the way. It allows air back into the
waste piping after the water has gone by. Lastly,
it allows sewer gases to escape outside through
a vent stack.
Where conventional venting is not practical,
mechanical devices can perform one of the vent
functions, allowing air into the waste piping to
prevent siphoning. These devices are known as
air admittance valves and automatic air vents.
These one-way devices also prevent sewer
gases escaping into the home when the plumb-
ing system is under positive pressure. Because
air admittance valves and automatic air vents do
not allow air to be pushed out through the vent,
they don’t perform all of the functions of a vent-
ing system.
Air admittance valves
Air admittance valves (AAVs) work with a sealing
mechanism that is lifted to let air into the drain
system when there is negative pressure. Posi-
tive pressure causes the mechanism to close so
that gases cannot escape into the home. These
are approved in many areas although they are
not as good as
conventional vent-
ing. AAVs are typi-
cally made of PVC
with an EPDM
seal and screened
vents. They are
generally mainte-
nance free. Indi-
vidual and branch
AAVs should be
at least 4 inches
above horizontal
drains. Stack type
AAVs should be
installed a mini-
mum of 6 inches
above the flood level of the highest fixture being
served. AAVs must be accessible. AAVs in
attics should be at least 6 inches above the attic
insulation. Some authorities recommend air
admittance valves on kitchen island fixtures to
reduce the risk of horizontal vent lines becoming
obstructed.
Automatic air vents
Automatic air vents are no longer approved
and should be replaced. Similar in function to
air admittance valves, these vents allow air to
be drawn in when the waste plumbing is under
negative pressure. This occurs when a vacuum
is being formed and the system is about to
siphon. These vents, when working properly,
can allow house air to be drawn in and break
the vacuum, preventing the siphon problem.
Air admittance valves are better
than automatic air vents
Automatic air vents are spring type devices
often referred to as in-line or cheater vents,
while air admittance valves have no spring. Air
admittance valves are more expensive and are
approved in many jurisdictions where in-line
Waste Plumbing:
Air Admittance Valves and Automatic Air Vents
By Alan Carson, Carson Dunlop & Associates
continued on page 35
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