by the last fixture tend to scour the vents of other
fixtures on the line.
When liquid wastes flow through a portion of
a vent pipe, the pipe is known as a WET VENT.
A LOOP VENT is the same, except that it
connects into the stack unit to form a loop. This
type may be used on a small group of bathroom
fixtures, such as a lavatory, water closet, and
shower, as shown in figure 8-22. The pipe for a
wet vent installation should be sized to take care
of the lavatory, water closet, and shower.
NOTE: The pipe for a wet vent installation
Figure 8-20.-Two fixture units sharing a common vent.
A COMMON VENT vents two traps to a
single vent pipe, as shown in figure 8-20. The unit
vent can be used when a pair of lavatories is
installed side by side, as well as when they are
hung back to back on either side of a partition
(as shown in the figure). A point to note is that
the waste from both fixtures discharges into a
double sanitary tee.
A CIRCUIT VENT serves a group of fixtures.
As shown in figure 8-21, a circuit vent extends
from the main vent to a position on the
horizontal branch between the last two fixture
connections. If more than eight fixtures are to be
vented, an additional circuit vent is to be installed.
In this type of vent, water and waste discharged
Figure 8-21.-Use of a circuit vent.
should never be under 2 in. in diameter when it
will be draining more than four fixture units. A
water closet should not drain into a wet vent.
As shown in figure 8-22, the lavatory should
be individually vented. This is necessary to
prevent loss of the trap seal through indirect
siphonage. Another point to note is that the
relatively clean water discharged from the lavatory
will tend to scour the wet vent, preventing an
excessive buildup of waste material in the vent.
Materials used in vent piping ordinarily
include galvanized pipe, cast-iron soil pipe, and,
at times, brass, copper, and plastic piping.
In all phases of the venting system, it is best
to use proper-sized piping. Remember that the
diameter of the vent stack or main vent must be
no less than 2 in. The actual diameter depends
on the developed length of the vent stack and on
the number of fixture units installed on the soil
or waste stack. The diameter of a vent stack
should be at least as large as that of the soil or
Figure 8-22.-Use of a wet vent.