by the last fixture tend to scour the vents of otherfixtures on the line.When liquid wastes flow through a portion ofa vent pipe, the pipe is known as a WET VENT.A LOOP VENT is the same, except that itconnects into the stack unit to form a loop. Thistype may be used on a small group of bathroomfixtures, such as a lavatory, water closet, andshower, as shown in figure 8-22. The pipe for awet vent installation should be sized to take careof the lavatory, water closet, and shower.NOTE: The pipe for a wet vent installationFigure 8-20.-Two fixture units sharing a common vent.A COMMON VENT vents two traps to asingle vent pipe, as shown in figure 8-20. The unitvent can be used when a pair of lavatories isinstalled side by side, as well as when they arehung back to back on either side of a partition(as shown in the figure). A point to note is thatthe waste from both fixtures discharges into adouble sanitary tee.A CIRCUIT VENT serves a group of fixtures.As shown in figure 8-21, a circuit vent extendsfrom the main vent to a position on thehorizontal branch between the last two fixtureconnections. If more than eight fixtures are to bevented, an additional circuit vent is to be installed.In this type of vent, water and waste dischargedFigure 8-21.-Use of a circuit vent.should never be under 2 in. in diameter when itwill be draining more than four fixture units. Awater closet should not drain into a wet vent.As shown in figure 8-22, the lavatory shouldbe individually vented. This is necessary toprevent loss of the trap seal through indirectsiphonage. Another point to note is that therelatively clean water discharged from the lavatorywill tend to scour the wet vent, preventing anexcessive buildup of waste material in the vent.Materials used in vent piping ordinarilyinclude galvanized pipe, cast-iron soil pipe, and,at times, brass, copper, and plastic piping.In all phases of the venting system, it is bestto use proper-sized piping. Remember that thediameter of the vent stack or main vent must beno less than 2 in. The actual diameter dependson the developed length of the vent stack and onthe number of fixture units installed on the soilor waste stack. The diameter of a vent stackshould be at least as large as that of the soil orwaste stack.Figure 8-22.-Use of a wet vent.8-16
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