Kitchen & Bath
Plumbers hate cutting holes in roofs for vent lines: My left
ankle is mostly stainless steel from cutting one too many of
them. Vent lines are also a nuisance inside the house, running
horizontally and vertically through floors, walls, and
ceilings, and then on into the attic, following codes —
and requiring holes — all the way. Not only can all these
holes compromise the structural integrity of the framing, they
can create potential fire paths.
But vents are required by code for a reason. When fluids flow
down an undersized drain line and completely fill it, air in
the drain line is shoved ahead of the water flow (assuming
there are no vents), creating positive pressure in front of the
water slug and negative pressure behind. This negative pressure
can create enough suction to siphon water out of a sink trap,
allowing sewer gas to enter the house through the now-empty
trap. And a drainpipe doesn't have to be totally filled with
flowing water to pull traps dry. Air tends to follow
fast-flowing water, so water rushing down a vertical drain line
can siphon water out of an adjacent horizontal branch.
More Than One Way to Vent
Individual vents placed within a few feet of each trap counter
pressure problems by allowing outside air to enter the drain
line. But there are situations where it is physically
impossible to install a vent line behind the trap — when
a long window is located behind a kitchen sink, for example, or
when the fixture is located beneath a structural beam that
can't be drilled, or when the kitchen sink or vanity is located
in an island.
Building inspectors always have a solution: expose the vent
line, move the sink, move the window, or change the framing.
It's also possible to design a complicated, labor-intensive
venting system near the fixture to compensate, as is
occasionally done for a kitchen island. But by far the simplest
solution is to install an air-admittance valve, or AAV. These
little miracle-workers almost make traditional vent lines
obsolete (see Figure 1).
valves can be used wherever a traditional vent would be
problematic, or simply to make fixture installation easier
during a remodel. Available in a range of sizes, AAVs can be
used to vent single fixtures, branch lines, and vent stacks,
but they don't eliminate the need for at least one vent to
AAVs aren't new. Developed in Europe back in the '70s, the
Studor Air Admittance System (Studor, 800/447-4721,
www.studor.net) was introduced in the
United States in 1988. Now, AAVs are accepted by virtually all
national building codes, including the SBCCI, BOCA, IRC, and
IPC plumbing codes. Under the IPC, AAVs are listed for
installation under Section 917; west of the Mississippi River,
they're covered under Section 301.2 of the UPC. While I've been
installing Studor's AAVs for quite a few years without a single
failure, Oatey (800/321-9532, www.oatey.com) and other manufacturers
offer them as well.
An AAV is simply a gravity-operated, one-way air valve that
allows fresh air to enter a drain system without allowing sewer
air out. You may have seen an AAV before and not known it. If
you've ever tripped over a white mushroom-shaped object up in
the attic, or seen a white cylindrical device under a kitchen
sink, that was probably an air admittance valve. AAVs cost
around $25 and up, depending on the size of the vent.
Don't confuse AAVs with smaller and cheaper mechanical vents.
Sometimes called "cheaters," these tubular, spring-loaded $5
vents are rated for only 1/2 DFU (drainage fixture unit), and
are not allowed under most building codes. To distinguish
between the two devices, look for the testing protocol —
ASSE 1051 or ASSE 1050, and perhaps NSF 14 — stamped on
the AAV's body. You'll also see code organization (such as the
SBCCI) approval stamps on the packaging.
Available in different sizes, AAVs can be used to vent an
individual fixture, a complete branch, or an entire stack of
vents. There are specialty AAVs rated for use outdoors and in
chemical environments, and even combination trap/AAV
assemblies. (Studor's compact version, the Combi-Siphon Plus,
is sold almost all over the world, but not in the U.S. You can
probably buy one, but if you are in an area governed by codes,
the inspector won't know what to make of it and won't pass
AAVs give you a lot of venting options, though a few
limitations do exist. For example, you can install an
individual AAV on each fixture and a larger AAV on the stack in
the attic — and, as long as there is at least one regular
vent to outside air, you're done. Each AAV has to be accessible
and have unimpeded airflow, so it can't be sealed inside a
wall. But if air and access is available — via a
removable louvered panel or screen covering, say — it can
be mounted there (Figure 2). AAVs must be installed vertically
(no more than 15 degrees out of plumb), and they can't vent
multiple fixtures when the fixtures are on different floors. If
placed in an attic, an AAV needs to be installed at least 6
inches above the insulation.
Figure 2.An AAV can be
mounted in a wall, but it needs to be accessible and have an
uninterrupted air supply (it can't be buried in insulation).
Some manufacturers offer special boxes for this purpose,
although they can easily be built on site.
If you're installing a new fixture, all you need to do is cut a
tee in the line behind the trap. You may need to offset the
rough-in pipe 4 inches to the side of the sink's standpipe
rather than bringing it in directly behind the standpipe. This
sends the trap over to the side, making room for the tee just
before it hits the back wall.
Next, on the upside of the tee, install a short 4-inch pipe
with a glue-on female threaded hub, and then screw in the
threaded AAV using Teflon tape. (Never use paste, as chemicals
in some pastes could destroy the threads.) AAVs can interface
with PVC or ABS, but with threads, you don't have to match
different plastics: They can screw into white PVC, black ABS,
cast iron, steel, or even copper. Studor's Mini-Vent —
the one I use most often — is rated for up to a 2-inch
vent and comes with a 1 1/2-by-2-inch reducing connector that
slips onto a 2-inch pipe.
For adding a fixture or two during a remodel, or for using an
AAV merely to eliminate running unnecessary vents during new
construction, you don't need to do any calculations. Just
install the AAV — you won't get into trouble by adding an
AAV along with the fixture (Figure 3).
Figure 3.In many cases,
it's easier to insert an AAV between a sink trap and a
drainpipe than it is to run a traditional vent, making the
connection with a few standard fittings (left) or with a
preassembled unit (right). When installed, the AAV's sealing
membrane needs to be at least 4 inches above the horizontal
pipe that it connects to.
If you're doing an entire house, you don't want to overtax a
particular AAV by putting too many fixtures on it (Figure 4).
Simply follow the AAV's guidelines. For example, Studor's
Mini-Vent can vent three DFUs on a 11/2-inch line (that's the
maximum DFUs you are allowed on that horizontal branch by code)
and six DFUs on a 2-inch line (again, the maximum amount of
DFUs allowed by code on that horizontal branch). Consult the
DFU chart — Tables 709.1 and 710.1(2) in the
International Plumbing Code, or the equivalent in the UPC
— to see what fixtures fit into those numbers. On a
3-inch horizontal branch, the Mini-Vent can handle up to 30
DFUs (drainage fixture
Minimum Size of Trap
Automatic clothes washer (residential)
Entire bath group (water closet, bidet, lav,
bathtub or shower)
Bathtub or whirlpool
Water closet (residential)
Drain Pipe Size (in inches)
(maximum DFUs allowed)
Vertical Stack —
more than three branch intervals
(maximum DFUs allowed)
1 1/4-1 1/2
vent more than one fixture, but their capacity must be matched
to the total DFUs on the branch line they vent. AAVs range in
capacity from 6 to 500 DFUs.
To vent a clothes washer with an AAV, I run a 1 1/2-inch pipe
up from a tee located about 4 inches behind the 2-inch trap,
terminating it in the washer box (Figure 5). That's where I
install the AAV; one box does it all. However, remember that
the horizontal pipe the trap dumps into must be at least 3
inches in diameter.
Figure 5.Fitting easily
inside a laundry box, a small AAV is a good choice for venting
a washing machine. This one connects to a 2-inch trap, which
drains into a 3-inch branch line.
Rex Cauldwell is a master plumber in Rocky
Mount, Va.; a frequent contributor to JLC; and the author of
several books, including Wiring a House.