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
Here's something to think about. Know why managers of large commercial
buildings have ALL the floors mopped routinely -- even in areas that
are never used? It's to keep the floor drains wet so they don't stink
(seriously). They also make floor drains with "drippers" built in that
connect to a water line. They drip to keep the trap wet.
I guess all I can add to this is to cut the pipe open, take a good
whiff, and see what you think.
BTW, what you are proposing is known to happen in larger applications,
but they use a mechanically ventilated pipe so there's no back draft.
Precisely. That's *exactly* what the trap is for. Many people believe,
mistakenly, that the trap is there to catch objects and stop them from going
down the drain. Nope. It's there for one reason: to keep sewer gas from coming
*up*. And that's why it's important for there to be water in the trap all the
How many times do you need to be told it's not a good idea, before that
finally sinks in?
Doug Miller (alphageek at milmac dot com)
SEWER ODORS are a problem. SWEETFILTER is the solution.
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Every city, town, etc. has obnoxious odor regulations.
Just tell your neighbors, friends, public officials, etc. that their
rooftop vent pipes and street manholes STINK and you have a solution at
You can makes hundreds of dollars each day selling and installing
I see you have a long thread about this, but I didn't
see one of the simplest answers. The sewer stack isn't
big enough. The vent fan needs a 4 inch line. For a
sewer vent plus a fan vent you need a 6 to 8 inch
line. I doubt if you have a sewer stack that large.
The proper way is to run the vent fan in a 4 inch
metal vent pipe to its own outside vent.
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