They use both. I believe this one prevents the house trap from being sucked
dry


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RBM (remove this) wrote:
It's a vent for the main waste line. You can seal around it, but don't
close
up the pipe, the system needs to breath


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It comes up from the floor, across the ceiling and through the wall of
the house, and bends outside the house, and is open at that end, Any
clue what its purpose is, my house was built in 1919, and I would kinda
like to seal it cause it brings in cold air.



Sounds like the old style vent lines that did not go above the roof.
Instead they came out the side of the house and had a cap that kept
things out, but let air flow.