Building Design and Construction Handbook - Merritt - Ventech!

14.46 SECTION FOURTEEN
To ensure that the vent will adequately protect the trap, plumbing codes generally
limit the distance downstream that the vent opening may be placed from the trap.
This distance generally ranges from 2 1 ⁄2 ft for a 1 1 ⁄4-in fixture drain to 10 ft for a
4-in fixture drain, but not less than two pipe diameters. The vent opening should
be located above the bottom of the discharge end of the trap (Fig. 14.11). In general,
all trapped fixtures are required to have an individual vent, although vents may be
eliminated under some exceptional conditions. The plumbing code should be reviewed
to determine where and how individual vents are to be installed.
To reduce the amount of piping required, two fixtures may be set back to back,
on opposite sides of a wall, and vented by a single vent (common vent). In that
case, however, the fixtures should discharge wastewater separately into a double
fitting with inlets at the same level.
A branch vent is a pipe used to connect one or more individual vents to a vent
stack or to a stack vent.
A wet vent is a pipe that serves both as a vent and as a drainage pipe for wastes
other than those from water closets. This type of vent reduces the amount of piping
from that required with individual vents. For example, a bathroom group of fixtures
may be vented through the drain from a lavatory, kitchen sink, or combination
fixture if such a fixture has an individual vent (Fig. 14.lla).
A battery of fixtures is any group of similar fixtures that discharges into a common
horizontal waste or soil branch. A battery of fixtures should be vented by a
circuit or loop vent. (Building codes usually set a limit on the number of fixtures
that may be included in a battery.)
A circuit vent is a branch vent that serves two or more traps and extends from
the vent stack to a connection to the horizontal soil or waste branch just downstream
from the farthest upstream connection to the branch (Fig. 14.11b).
A loop vent is like a circuit vent but connects with a stack vent instead of a
vent stack (Fig. 14.11c). Thus, air can circulate around a loop.
In some instances, conventional venting methods cannot be applied, such as with
island sink fixtures. Some codes allow the use of air admittance devices, commonly
known as quick vents. These devices allow air to enter the vent system while
preventing sewer gasses from escaping.
Soil and waste stacks with more than 10 branch intervals should be provided
with a relief vent at each tenth interval installed, starting with the top floor. A
branch interval is a section of stack at least 8 ft high between connections of
FIGURE 14.11 Venting of waste branches: (a) wet venting of bathtub drainage pipe; (b) circuit
venting, and (c) loop venting of a battery of plumbing fixtures.