I found a bathroom fan being vented into the DWV Stack system in the attic. I was always under the impression that nothing should vent into the DWV system. Has anyone seen this done before? Is it allowed?
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No, write hard on that one.
Also, that would be a good picture for the WTF thread.
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Someone put a fair amount of work into doing something pretty dumb. No dice. 
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So sewer gases come up the vent and exit into the bathroom. I bet it smells like an outhouse.
When the bathroom fan is on however, it would provide as much supply air as the vent needs. Kinda like a turbo right up until you start getting a bowl full of bubbling turds.
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You can always find someone that can do it cheaper. You would be hard pressed to find someone to do it worse.
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really?
DWV system does not include bath ventilation.
BTW, that would be a nice drain point for all the rain water that enters the vent.
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They saved some money on a reducer by fabricating a transition in the duct itself.
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Put’s a new meaning for a Power Vent! We have found those a few times in our area and every single time the owner of the home had worked at the GM Assembly Plant which was formally the Saturn car plant! I have no idea of the correlation with the plant but my guess is they were board engineers that all retired and had nothing else to do. They also like to seal weep holes up with silicone!
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Hay Scott, I was doing Thermal Imaging on a couple houses behind the Catholic Church across from Bass Pro on Briley, after the big flood. A couple houses up there on the hill were flooded inside walls and the basement because of water entering weep holes! The houses were built after the period when weep holes were not required for a period of time. When they found that they were actually needed, they required weeps all over the place. Over and under windows, doors, at floor plates, even walls without openings. These were three-story walls at the back of the houses, facing the storm.
I may have been responsible for this fan vent design. I think this guy was an engineer at Saturn/GM! 
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I think it is covered in M1507.
Air exhaust from the bathroom must be sent outdoors, not indoors. It cannot move air to a crawlspace or attic.
Where is it being sent in the photo? Inside?
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Bathroom fans don’t have a damper installed until termination. I can just imagine the smell in the bathroom when the fan isn’t running.
A nice gentle breeze on the outside will push that sewer guest directly into the bathroom.
We see bathroom vent fans all the time that have a flapper baffle/damper built in the unit. But they will not stop sewer gas, they are only designed to limit air being pushed back in from outside.
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The fans I have all have built in “dampers” within the main unit and I agree that those would not keep fumes from seeping back through.
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Sent to the attic and routed to the Main Plumbing Stack Vent. This will certainly cause an imbalance of air pressure.