Plumbing Code - Can I tie my bathroom exaust fan into the main plumbing vent to the outside.

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johnnymo wrote:

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.
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johnnymo wrote:

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.
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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 time.

How many times do you need to be told it's not a good idea, before that finally sinks in?
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Doug Miller (alphageek at milmac dot com)
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wrote:

He was waiting for YOU to stop by and tell him. He didn't believe anyone else.
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You really should see a therapist, Joe. This fixation you have on me isn't healthy.
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Doug Miller (alphageek at milmac dot com)
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wrote:

My fixation? 4-5 hours ago, half a dozen people told him his idea was lame. You needed to chime in and say the exact same thing?
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I see you're still having trouble with reading comprehension -- I responded to his question about the trap.
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Doug Miller (alphageek at milmac dot com)
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johnnymo wrote:

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.
Bill Gill
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