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I have a tiny bathroom for the toilet, perhaps 83 cu ft adjacent to the master bathroom for the shower and tub, perhaps 1266 cu ft. Currently only the toilet has a vent with a fan. It’s being removed in the remodel. I’m adding an exhaust register to the main bathroom and I’d like to vent both rooms with a single 8” 210CFM in-line duct fan.

Since the toilet is so much smaller, should I restrict its airflow with a smaller pipe ex 3” vs the main bathroom 6”? Then have them meet at a wye before the in-line fan which would exhaust out via a 6” pipe?

Obviously I want more air to be pulled from the larger space but want to do that while maximizing the in-line fans capabilities.

Lastly, would the damper, to restrict backdraft, go after the fan or before?

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  • If I understand, you want to run the toilet vent fan through the shower vent blower motor? Don't do that, have them meet in a "Y" after both blowers. – FreeMan Jun 2 '20 at 18:39
  • Only one in-line fan for both rooms and removing the existing toilet fan. So the in-line would pull air from both rooms. – DIY Tom Jun 2 '20 at 18:56
  • ah, gotcha! You may want to edit your question to help make that more clear. Some formatting and line breaks will help with that, too. – FreeMan Jun 2 '20 at 19:01
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    Thanks! Gave it a touch up! :) – DIY Tom Jun 2 '20 at 19:13
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There are some companies that make zoned bath ventilation systems if you need such precise control.

One fan unit, multiple register units at each location that has a damper control at the register. Depending which zone is turned on, the corresponding damper at that register will open to allow airflow. For example, American Aldes Vent Zone

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    Perhaps you could discuss more about how systems of the nature you describe function, and how they'd solve OP's issue? – ThreePhaseEel Feb 13 at 22:52
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Yes, that is perfectly do-able. Since the fan is AFTER the wye, no airflow issues. However, you will be having open vent between the toilet and shower area. So smells from the toilet area can migrate to the shower area (especially when the fan is off).

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