Sink
Traps
by Frank G. Ross
One of the
innovations that make indoor plumbing possible is sink traps.
These curved drain pipes prevent sewer gasses and odors from
entering the house. A U-shaped section of the drain pipe fills
up with water, forming a seal. The water blocks the movement of
sewer gas and prevents it from flowing up through the trap, the
sink drain and into the room. Every plumbing fixture - sinks,
tubs, washing machines, and showers - should have a proper trap.
Toilets have traps integrated as part of their design. Sink
traps are the most visible since they are usually located under
the sink while a tub trap will most likely be in the wall.
Drains that
penetrate a wall have a P trap while those that go through the
floor have an S trap. Most municipalities no longer permit the
installation of S traps in new construction. One reason is that
they are very prone to clogging. However, during my home
inspections over the years, I have seen many of them in older
houses and in homeowner-built bathrooms. They are also still
available for purchase. Certain drum-styled traps which have
moving parts inside are also discouraged or banned because if
they become corroded inside they no longer function.
Another problem
with an S trap is that the water seal may be lost due to
siphonage.
The water held by the trap is theoretically replaced each time
the fixture is used. In the case of S traps, when water flows
from the sink through the drainpipe it also removes the air from
that pipe which causes negative pressure in the drain line. The
negative pressure can siphon the water from the trap. The water
seal is lost and sewer gas can flow into the room. A quick fix
for this problem is to run the water for 5 seconds, just enough
to fill the trap again. Siphoning can occur even when the trap
is well-vented.
The design of a P trap differs from an S trap in that the horizontal
portion of the trap is connected to a plumbing vent pipe which
extends through the roof, so there is always atmospheric pressure on
the down side of the trap. Siphonage is unlikely when a P trap is
used.
It is possible to
retrofit under sink drain plumbing removing the S trap and replacing
it with a P trap. The U-shaped portion of the S is replaced with
the P trap while the arch of the S is replaced with a type of T
capped by an Air Admittance Valve. An Air Admittance Valve (AAV)
is a one-way valve that allows air into a plumbing system when there
is water flow to allow drainage. Because the valve closes from
internal air pressure, it seals under no-flow conditions effectively
containing sewer gasses. The valve will not open if there is
positive internal pressure in the drainage system. Although AAVs
are accepted in all current plumbing codes and by the American
Society of Sanitary Engineers (ASSE), there are U.S. localities that
specifically prohibit the use of AAVs. Check local codes before
installing one.
Obstructions in the venting system or the lack of a vent can also
cause siphoning and empty a sink trap. Obstructions might include
birds or other small animals, leaves, twigs, insect nests or other
debris or even snow. The lack of a vent would probably only apply
to an older home. Sewer gas may also enter a home through a
little-used drain if the water has evaporated from the trap, such as
a basement floor drain. The solution is the same as for a siphoned
trap: pour water into the drain and recreate the seal.
Frank Ross is a Certified Utah Home Inspector for Pillar to Post
Professional Home Inspection and a full member of the National
Association of Certified Home Inspectors and a member in good
standing with the Utah Better Business Bureau. Readers may contact
Mr. Ross at (435) 867-6400 or
frank.ross@pillartopost.com
