Waste pipes are vented to the outside to balance the air pressure in various branches and to dilute any gases present. The availability of air prevents back pressure and protects traps against siphonage.

Types of Vents

The main vent is the principal artery of the venting system. It supplies air to branch vents, which, in turn, convey it to individual vents and wastewater pipes.
Every building should have at least one main vent stack. It should extend undiminished in size and as directly as possible from outdoor air at a level at least 6 in above the roof to the building drain. The main vent should be so located as to provide a complete loop for circulation of air through the wastewater-removal system.
As an alternative to direct extension through the roof, a vent stack may be connected with a stack vent, if the connection is made at least 6 in above the floodlevel rim of the highest fixture.
A stack vent is the extension of a soil or waste stack above the highest horizontal drain connected to the stack. This vent terminates above the roof.
A vent through roof (VTR) is any vent that extends through the roof to allow escape of sewer gases and to equalize pressures in the drainage system to prevent siphonage from trap seals. In colder climates, a VTR should be at least 4 in in diameter to prevent blockage from formation of frost and should terminate at least 12 in above the roof, but higher if the VTR is installed in regions with high snowfall rates.
An individual vent, or back vent, is a pipe installed to vent a fixture trap and is connected to the venting system above the fixture served or terminated outdoors.

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 21⁄2 ft for a 11⁄4-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 horizontal branches. A relief vent provides circulation of air between drainage and venting systems. The lower end of a relief vent should connect to the soil or waste stack, through a wye, below the horizontal branch serving a floor where the vent is required. The upper end of the relief vent should connect to the vent stack, through a wye, at least 3 ft above that floor. Such vents help to balance the pressures that are continuously changing within a plumbing system.

Slopes and Connections for Vent Pipes

While the venting system is intended generally to convey only air to and from the drainage system, moisture may condense from the air onto the vent pipe walls. To remove the condensation from the venting system, all individual and branch vent pipes should be sloped and connected as to conduct the moisture back to soil or waste pipes by gravity.

Sizing of Vent Pipes

Fixture units (Art. 14.19) are also used for sizing vents and vent stacks (Table 14.12). In general, the diameter of a branch vent or vent stack should be one-half or more of that of the branch or stack it serves, but not less than 11⁄4 in. Smaller diameters are prohibited, because they might restrict the venting action.

Combined Draining and Vent Systems

These offer the possibility of considerable cost savings over the separate drainage and venting systems described in Art. 14.20.1.
One such system, introduced by the Western Plumbing Officials Association, employs horizontal wet venting of one or more lavatories, sinks, or floor drains by means of a common waste and vent pipe adequately sized to provide free movement of air above the flow line of the pipe. Relief vents are connected at the beginning  and end of the horizontal pipe. The traps of the fixtures are not individually vented.
Some building codes permit such a system only where structural conditions preclude installation of a conventional system. Where this combined drainage and vent system may be used, it may require larger than normal waste pipes and traps. Each of the model codes has different requirements for this type of system and, therefore, the code in effect must be carefully reviewed during the design process.
The Sovent system is another type of combination system. It requires drainage branches and soil stacks, with special fittings, but no individual and branch vents and no vent stacks.
The system has four basic parts: a soil or waste stack with a stack vent extending through the roof, a Sovent aerator fitting on the stack at each floor, horizontal branches, and a Sovent deaerator fitting on the stack at its base and at horizontal offsets (Fig. 14.12). The aerator and deaerator provide means for self-venting the stack. In a conventional drainage system, a vent stack is installed to supply air to vent pipes connected to the drainage branches and to the soil or waste stack to prevent destruction of the trap seals. In the Solvent system, however, the vent stack is not needed because the aerator, deaerator, and stack vent avoid creation of a strong suction.

The aerator does the following: It reduces the velocity of both liquid and air in the stack. It prevents the cross section of the stack from filling with a plug of water.
And the fitting mixes the wastewater from the drainage branches with the air in the stack.
The deaerator separates the airflow in the stack from the wastewater. As a result, the wastewater flows smoothly into a horizontal offset or building drain. Also, air pressure preceding the flow at 90 turns is prevented from rising excessively by a pressure relief line between a deaerator and a stack offset or the building drain to allow air to escape from the deaerator.
An aerator is required on the stack at each level where a horizontal soil branch or a waste branch the same size as or one tube size smaller than the stack discharges to it. Smaller waste branches may drain directly into the stack. At any floor where an aerator fitting is not required, the stack should have a double in-line offset, to decelerate the flow (Fig. 14.12). No deaerators are required at stack offsets of less than 60.

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