Top comes out on the roof:

Not sure what happens below. Haven’t gone under the house to inspect. Seems like a vent that travels through the wall space, but unclear why it would do that.

Any ideas what its purpose is?

      • Iconoclast ( Iconoclast@feddit.uk ) 
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        2 months ago

        In europe the standard drain pipe outside diameter is 110mm but the vent pipe sits over it so it’s obviously thicker. However this is to create an insulating air gap between the vent and the drain pipe itself to prevent it from freezing here in a cold climate. Depending on where you live this might not be a thing.

        Drain vent still remains my guess for what that is. Every house has atleast one but it’s not uncommon for there to be 2 or 3 of them.