of nickel or chrome-plated brass, malleable
galvanized; cast iron, other metal alloys, and plastic.
The P-trap is used for fixtures suspended from the
walls or supported on pedestals, for instance,
lavatories, sinks, and urinals. At times the P-trap may
also be suitable in showers, baths, and installations that
do not waste large amounts of water.
When using a P-trap for fixtures suspended from
the wall, you should install it as close to the fixture as
possible. Be careful not to install a vertical leg that is
too long between the trap and the fixture. It is also
important for the horizontal leg connection to the
waste system to be short for ventilation purposes.
To prevent the siphonage of a trap seal in fixture
traps and allow gravity flow of drainage, you must let
atmospheric air from outside the building into the
piping system to the outlet (or discharge) end of the
trap. The air is supplied through pipes called VENTS.
This air provides pressure on the outlet end of the seal
equal to pressure on the inlet end.
Atmospheric pressure at sea level is about 14.7
pounds per square inch. This pressure remains
virtually constant on the inlet end of the water seal.
Obviously, a greater or lesser amount of pressure on
the outlet end of the trap seal forces the water in the
direction of least resistance. Since the air supplied by
the vent to the outlet end provides a pressure equal to
that at the inlet end of the trap, the trap seal cannot
escape through siphonage.
All vent systems should be provided with a main
vent or vent stack and a main soil and waste vent. A
main vent may be defined as the principal artery of
the venting system, and vent branches may be
connected to the main vent and run undiminished in
size as directly as possible from the building drain to
the open air above the roof. The MAIN SOIL AND
WASTE STACK, as shown in figure 3-8, is installed in
a vertical position.
The term main soil and waste vent, or soil stack
vent, refers to the portion of the stack extending above
the highest fixture branch. In figure 3-8, this vent
extends through the roof. Actually, it is an extension of
the main soil and waste stack.
Common Types of Vents
Various types of vents are used in the ventilation of
fixtures; even in the best of installations, you may find
several different types of vents. The selection of a
particular type of vent depends largely on the manner
in which the plumbing fixtures are located and
grouped. Some of the common types of vents you may
use frequently in your work are mentioned briefly
below. An INDIVIDUAL VENT, also known as a
BACK VENT, is a vent that connects the main vent
with the individual trap underneath or behind a fixture.
This method of venting is shown in figure 3-35. When
you install two or more fixtures on an individual vent
basis, ensure the leg (see illustration) connecting
individual vents to the main vent is large enough to
carry the total load.
A COMMON VENT vents two traps to a single
vent pipe, as shown in figure 3-36. The unit vent can
be used when a pair of lavatories are installed side by
side, as well as when they are hung back to back on
Figure 3-35.Individual (or back) venting.
Figure 3-36Two fixtures unit vented.