Any guess as to what this hose connecting to a plumbing vent is? I couldn’t follow where it came from. It doesn’t look like one to me, but it cant be a condensate pipe. HVAC is on the floor lower in a closet.
Maybe it runs to a condensation pump, or water softener.
You can’t tap or drill a hole in a DWV pipe, so it shouldn’t be there anyway.
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Drain wet line?
Cheers
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huh, never heard of that. thanks Doug!
Ok maybe not in the USA. Here in Ontario a small hose is attached to a faucet, usually the laundry room and it is inserted into a drain line or stack. The purpose is to maintain water in the basement trap.
Cheers
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Ahhh right, “trap primer” is what I’m familiar with it being called. Thanks for the reminder.
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I’ll echo Dominic. Around these parts, 9/10 it is for a water softener. 1/10 a condensate pump.
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Might be an RO (reverse osmosis) waste water discharge line. They are typically made of black tubing.
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Unless there is mechanical water pressure, that thin black hose draining into DWV atmospheric vent pipe will be backed up with water. Look at the radius. It slopes up then down into the DWV pipe.
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Unless you can find the point of origination every answer is just that, a guess.
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Thinking of putting this in my narratives… 
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I can’t tell you how many times a furnace condensate line or something similar in a condo disappears into the sheet rock. I just report that final discharge method and location is not visible. For all I know it’s dumping into the ceiling below.
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So true. Report what we see and not why. I know we all want to learn and understand “why”, but sometimes it just not feasible and beyond the SOP.
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Need more information. What equipment was in the attic space.
Guessing is prohibited when reporting in my world.
I’ve see condensate lines tapped in and water filter/softener systems tapped in. I have also seen dehumidifiers. Last one was water treatment.
Around here a water softener drain needs trap. 
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Oh, here too, Larry! A trap & an air gap.
Just want to help identify the item for accurate reporting of deficiencies.
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