Or less the low end of the drain is not flooded and

If the drain is 5 ft. or less, the low
end of the drain is not flooded, and air can get in.
Without an effective plumbing drain vent, a vacuum forms in
the pipe between the water going down the drain and the water
that needs to stay in the trap.
 A siphon is created and the atmospheric air pressure pushes
the water out of the trap to satisfy the vacuum. This allows
sewer odors to back up into the house. The gurgling sound is
air forcing its way through the water in the trap.

If the drain pipe (trap arm) is longer than 5 feet
distance from the vertical plumbing vent stack, or
the plumbing drain pipe slope is more than ¼ inch
per foot, a 1 ¼ inch diameter pipe will flood.
That’s the same as having no vent. The illustration
below shows that air from the stack can’t get into
the trap arm, and a siphon results.

TYPES OF
PLUMBING VENTS

Individual vents
Individual vents serve
individual fixtures.
This vents only one fixture but may connect
into an other vent that extend to the open air.
an individual vent should be at least one-half
the size of the drain it vents (no less than 1
1/4", 32 mm)

Relief vents
Relief vents provide
additional air to the drainage
system where the primary
vent is too far from the
fixture.
a relief vent should be at least one-half the size
of the drain it vents (no less than 1 1/4", 32
mm)

Circuit vents
Circuit vents are used with
more than one fixtures. A
circuit vent is installed
before the last fixture
extend up to open air or
connects to an other vent
that extend to the outside
a circuit vent should be at least one-half the
size of the drain it vents (no less than 1 1/4",
32 mm)

Non-Conventional
Venting Methods

Island Fixture Venting
 This is a specific method for venting an island sink, one limited to sinks and
lavatories. Residential kitchens sinks with a dishwasher waste connection, a
food waste grinder, or both, in combination with the kitchen sink waste, shall
be permitted.
 The island fixture vent shall connect to the fixture drain as required for an
individual or common vent. The vent shall rise vertically to above the
drainage outlet of the fixture being vented before offsetting horizontally or
vertically downward.
 The vent or branch vent for multiple island fixture vents shall extend to a
minimum of 6 inches (152 mm) above the highest island fixture being vented
before connecting to the outside vent terminal.
 The lowest point of the island fixture vent shall connect full size to the
drainage system.
 The connection shall be to a vertical drain pipe or to the top half of a
horizontal drain pipe. Cleanouts shall be provided in the island fixture vent to
permit rodding of all vent piping located below the flood level rim of the
fixtures. Rodding in both directions shall be permitted through a cleanout.