A VENT (pipe) allows gases in the sewage drainage system to discharge to the outside. It also allows sufficient air to enter, reducing the air turbulence in the system. Without a vent, once the water is discharged from the fixture, the moving waste tends to siphon the water from the other fixture traps as it goes through the pipes. This means that the vent piping must serve the various fixtures, as well as the rest of the sewage drainage system. The vent from a fixture or group of fixtures ties in with the main vent. A MAIN VENT is the principal artery of the venting system to which vent branches maybe connected and run undiminished in size as directly as possible from the building drain to the open air above (fig. 8-19). The MAIN SOIL AND WASTE VENT or VENT STACK, installed in a vertical position, refers to the portion of the stack that extends above the highest fixture branch, through the roof, and to the exterior of the building.

Various types of vents are used in the ventilation of fixtures. The selection of a particular type depends largely on the manner in which the plumbing fixtures are to be located and grouped. An INDIVIDUAL VENT, also known as a BACK VENT, connects the main vent with the individual trap underneath or behind a fixture. This method of venting is shown in figure 8-19.

Figure 8-19.-Typical stack and vent installation.

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.