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Just opened a hole in my old house to replace some plumbing. I can see where the main drain/sewer stack comes down from the bathroom, but in parallel there's another galvanized? 2"? pipe coming down that T's into the main stack and also continues into the basement where it eventually goes into the slab. Any idea what this is for?

thanks -

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Hard to tell without seeing it, but it sound like it might be forming either a revent or a loop vent for something (maybe a floor drain in the basement?):

enter image description here

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