Ideas on this plumbing "vent"

Found in attic. I can see it comes from basement but basement is finished. Bathroom fans exhaust to exterior. Forced air furnace is direct vented correctly. Gas water heater is vented correctly. It’s not for kitchen range. Split level home. Used to be electric baseboard heat. Added furnace supply’s and one return at furnace. Finished basement so cant see where it begins. Homeowner has lived there for 15 years and is a commercial plumber. He isn’t sure what its for either. Just curios if anyone has an idea. thanks

Ryan, it could be many things if it’s not abandoned. It is all going to be just a good guess from here. It should be investigated/traced on site.

What is the diameter of the pipe? What is directly below it in the basement? Any sewer ejectors, confined utility rooms? Did you find all terminations for direct vented appliance?

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4” pipe. Here’s a guess…,passive radon mitigation where it is terminated below the slab with no fan.


It’s 4". Lower level hallway in the basement is directly below it. No sewer ejectors. The utility room is close to there but, it doesnt appear there. It’s all finished space, even the utilty room. Yes all direct vented appliances are terminating correctly.

I like the idea! House was built in 1991. I’ve never seen a passive mitigation system around here… or maybe i just did lol

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Agree with Martin.

The staining on the insulation looks like the inside of the pipe on almost every mitigation termination.

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The only other thought I would have is it is a vent for the gas water heater make-up/combustion air…But very well could be one of the other “Suspects” including a plumbing waste vent. Those I see stubbed up into the attic are put in by a plumber, I suspect during a remodel with the intention a roofer or someone else is going to extend it out through the roof at a later time. but normally a plumber would not add the turn down. I do commend you for trying to determine what the crap it is by eliminating a number of possibilities.

If it is it’s wrong. The vent pipe can’t terminate in the attic. Passive or active.

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Lower level hallway directly below it… so the ceiling of the hallway is between it and the open space below?