A dry vent begins at the point where water or sewage drains
into the drain line and goes up to the vent to atmosphere. In
a dry vent, nothing will pass through it but air under normal
conditions (a backed up sewer line may cause sewage to back up
into it, but that is not normal conditions).
A wet vent is one that has sewage draining through a part of
the vent. You will sometimes see these in older homes where
wet venting was legal at the time that they were built. In a
wet vent, the actual drain line will be used to get to a vent,
usually for another fixture. You may see this in bathtub
drains or where there is a window or other obstruction to a
dry vent. It is allowed for situations like islands with
sinks because it is the only way to vent them without having a
pipe running out of the top of the island and through the
ceiling. See the websites for explanations.
If you have Macromedia flash, you can see one here:
Here is another explanation of it:
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