Air-admittance valves can be used wherever a traditional vent would be problematic, or simply to make fixture installation easier during a remodel. Available in a range of sizes, AAVs can be used to vent single fixtures, branch lines, and vent stacks, but they don't eliminate the need for at least one vent to outside air.
Air-admittance valves can be used wherever a traditional vent would be problematic, or simply to make fixture installation easier during a remodel. Available in a range of sizes, AAVs can be used to vent single fixtures, branch lines, and vent stacks, but they don't eliminate the need for at least one vent to outside air.

Plumbers hate cutting holes in roofs for vent lines: My left ankle is mostly stainless steel from cutting one too many of them. Vent lines are also a nuisance inside the house, running horizontally and vertically through floors, walls, and ceilings, and then on into the attic, following codes — and requiring holes — all the way. Not only can all these holes compromise the structural integrity of the framing, they can create potential fire paths. But vents are required by code for a reason. When fluids flow down an undersized drain line and completely fill it, air in the drain line is shoved ahead of the water flow (assuming there are no vents), creating positive pressure in front of the water slug and negative pressure behind. This negative pressure can create enough suction to siphon water out of a sink trap, allowing sewer gas to enter the house through the now-empty trap. And a drainpipe doesn't have to be totally filled with flowing water to pull traps dry. Air tends to follow fast-flowing water, so water rushing down a vertical drain line can siphon water out of an adjacent horizontal branch.

Individual vents placed within a few feet of each trap counter pressure problems by allowing outside air to enter the drain line. But there are situations where it is physically impossible to install a vent line behind the trap — when a long window is located behind a kitchen sink, for example, or when the fixture is located beneath a structural...

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