replying to johnnymo, Attila wrote:
Exhaust fans have a flap door, so air can move only one direction. Also keeps
bugs out!
So I don't understand how sewer gas can go down into the exhaust fan. It can
open the door against the reversed airflow?
If the air duct from the exhaust fan connected into the vertical main sewer
vent which is 3 or 4 inch pipe with a T fitting, then any condensation from
the moist air forced out by the fan is just dripping down into the sewer!
When your fan is working, it creates positive pressure in the vent system
which is even help the water flow down if somebody flash the toilet or drain
the bat tub. It would act like the turbo charger on vehicles! If the fan do
not work at the time, when water is draining, the draining water creates
negative pressure in the vent so it sucks air from outside as usual and may
some air from your bathroom too, through the exhaust fan.
I can't think of a scenario, when a sewer gas rising up inside the main vent
would make a major U turn down towards the exhaust fan, open the flap door,
and sneak into your bathroom!
In a windy day you can see the water in your toilet moving up and down, cause
the pressure change in the vent pipe moving the trap water in the P-trap.
Because the exhaust fan have a flap door allowing air flow only one direction,
the sewer gas can't flow back downwards, passing the one way door!
Its act like the air admittance valves!

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