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1. 
   Riversong | Jun 01, 2010 01:04pm | #1

   Current model plumbing and residential codes allow air admittance valves (AAVs) as a substitute for fixture vents or branch vents (though some local jurisdictions may still prohibit them), but still require at least one vent stack to terminate outdoors and in some cases require a relief vent in addition to a branch AAV.

   Originally, they were designed to vent single fixtures, such as island sinks, and for renovations in which direct venting was prohibitive. Now they are more broadly allowed to reduce pipe runs and roof penetrations.

   But through-the-roof vent penetrations don't need to be an energy loss if they are properly sealed at the thermal envelope boundary (typically the ceiling). Because plastic pipe has a high coefficient of expansion, sealing with caulk or spray foam is usually only a temporary solution. Instead, use a roof flashing sealed above the ceiling. With no UV degradation to the rubber boot, it should last the life of the house.

   While AAVs are required to be tested to 500,000 cycles (approximately 30 years of use), they are still a mechanical device that's subject to failure. And, since the purpose of venting is to maintain water traps and prevent toxic and flammable sewer gases from entering the living space, a passive vent is almost always more reliable than a mechanical one.

   A passive, code-approved alternative to an island sink AAV is the Chicago Loop http://en.wikipedia.org/wiki/Chicago_Loop_(plumbing), and your code official may allow you to run a system vent out the gable rather than the roof if you're concerned about roof leaks.