Figure 8-20.-Two fixture units sharing a common vent.

Engineering Aid 3 - Beginning Structural engineering guide book
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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 waste stack. Figure 8-22.-Use of a wet vent. 8-16

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