Connection Guest Column
Running Vent Lines: Vent Line Theory and Technology Have
Evolved Substantially Over The Years
By Bob Beall, Mr. Rooter Plumbing
to Bob Beall, Mr. Rooter Plumbing president of the Northeast
Ohio and the Southwest Pennsylvania area, the purpose of a
vent line is to prevent the water in the traps from being
pulled out by a vacuum caused when rapidly running waste
water flows down a pipe.
“Vent line theory and technology have evolved substantially
over the years,” says Mr. Rooter. Early system design called
for one vent for every fixture. Then we found out we could
have a common vent system for several fixtures and vent them
with a single pipe. “The latest innovation,” says the most
referred plumber, “is the air admittance valve (AAV), which
replaces individual vent lines running through the roof."
AAVs are accepted by all the accredited agencies but are
still prohibited in some localities. If codes prohibit their
use in your area, it will be necessary to run an individual
vent from a fixture or install drainpipe from the fixtures
to a line sized to prohibit the formation of a vacuum. Beall
says, “In some instances a very expensive loop of pipe can
be created, as some do for an island drain in a kitchen.”
WORKING AROUND OBSTRUCTIONS
Tip #1 Window behind a sink will not allow vent line to be
installed, and can’t go around sink.
Tip #2 Use 2-inch drain line in a wall from sink drain to
3-inch main drain.
Tip #3 If you cannot run an individual vent because of a
window, use an over-sized drainpipe. Because the pipe can’t
fill with water, it won’t siphon the trap or
Tip #1 Window behind sink will not allow vent line to be
installed, and line can’t be run around window.
Tip #2 AAV
Tip #3 1 1/2-inch drain line.
Tip #4 Install an AAV in the fixture drain line before the
line enters the wall. Such an installation avoids the
necessity of running a vertical vent line.
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INSTALLING A FIXTURE
Tip # 1 Before running a vent line horizontally, the
vertical vent section must travel at least 6 inches above
the flood rim of the fixture it is venting. This keeps
sludge from setting in the vent line in case of a backup.
VENTING WITH AN INVERTED T-FITTING
Tip #1 Install a sanitary T-fitting upside down to allow the
vent gas to flow upward and out the vertical vent more
Tip #2 Branch the vent line to the roof.
Tip #3 Sewer gasses easily follow the interior curvature of
the upside down T-fitting.
Tip #4 Add upside-down sanitary T-fitting and then vent to
the main drain.
VENTING IN JOIST SPANS
Tip #1 Venting within the Joist Span: Use a street 90 to
point the pipe to the nearest wall to make it easy to vent a
Tip #2 Venting outside the Joist Seam: Extend the inlet pipe
and use two 90s to get to the vent wall.
AVOIDING VENT CLOGS
Tip #1 Never take off for a vent air beyond 45 degrees from
vertical. Below 45 degrees from vertical, a vent line can
plug up with water and debris.
BONUS TIP: Instead of running expensive revent loops,
install an AAV in the drain line at the fixture. Very large
AAVs are available on the market today and can vent an
entire branch (a pipe off the main line going to several
fixtures) at one time.