bathroom fan questions

I would like to add a bathroom exhaust fan in our second floor bathroom. I wondered if I could just tap into the exhaust stack (galv. standard 3" pipe) of the second bathroom fan with a "Y" (upside down ofcourse) connection rather then put another hole in my roof? another option would be to just vent it out the soffit, but not the preferred method I know.
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The "Y" is a bad idea and not generally permitted by codes or inspectors. It will allow air to blow from one bathroom into the other when only one fan is on and will likely not vent well when both are on. There are fittings with grilles for venting out the bottom of the soffit. There is generally a limit on how close this can be to a window. Do not vent into the attic or other enclosed space as moisture and condensation will likely cause extensive damage. Don Young

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You sure about that being not permitted or causing trouble?? Didn't bother the installers at my house to hook the gas furnace and gas water heater together with a "Y" and the inspector didn't mind either. I figure a Y for bathroom fans a Y would be a lot less problem than a furnace and a water heater.
Don Young wrote:

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Gas furnace and water heater vents emit hot air, which are generally sent up a common chimney, no problem. Two fans are a bit different, don't you think? They work by fan pressure, not convection. The air is not hot and doesn't have to go up, it can just as easily come back down into the other bathroom.
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I've seen a setup where two bathrooms had their exhaust fans work in unison from the fan switch in either bathroom - like a 3-way light switch. Whether this works as well as two separate exhaust vents is questionable.
FG

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