johnnymo wrote:
Thanks Grandpa.

So, what is the difference in hooking up a sink to the side of the
sewer vent than hooking up a piece of flex hosing that goes down.

Maybe the trap? Is it b/c there is always water/gunk in the trap not
allowing the gases to go up through the sink?

My exhaust fan hose is always going to be dry so that would be a
problem. Hmm....maybe not such a good idea?

That would be the exact reason, sink drains have traps to prevent sewer
gas from entering the room.

The fan connection to the DWV vent pipe not only violates code(s), its
a plain Bad Idea as stated by others. Aside from a low pressure
conditon in the building (think kitchen exahust fan, opening a well
sealed outside door with all the windows closed, the aforementioned
fireplace or wood stove) causing sewer gas to be drawn in when the fan
is not operating, it is possible (though less likely,) that with the
bathroom fan operating, it is conceivable that the positive pressure
introduced by the fan could have an adverse affect on one of the drain
trap's contents, should the vent outlet become (partially or fully)
obstructed, say be snow. I admit, this is a long shot, but you are
looking for reasons why this is a Bad Idea, that's another.

I'll echo what a previous poster said - make sure the fan exhaust makes
it outside. Don't let it loose inside the attic. You'll be asking for
moisture trouble.