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Old 11-14-2010, 04:09 PM   #16
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Can I use one roof vent for 2 bathroom exhausts fans?


Venting bath fans into a ventilated attic was legal under the 2000 IRC, but the 2003 IRC changed that, requiring that the fans vent directly outdoors. Putting the vent pipe by a roof vent is not "directly outdoors", and an actual penetration of the wall or roof is necessary to meet code.

There is nothing in the code that prohibits interconnecting bath fans into a common or shared vent. Code's a minimum standard, and I agree that it is less desirable than individual venting.

Another consideration is condensation of the vent in a cold attic. Warm air will cause the vent to sweat, sometimes causing water to run down the pipe and drip on insulation, celings, etc. Insulating bath fan vents with flex duct isn't a bad idea.

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Old 11-14-2010, 05:15 PM   #17
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Can I use one roof vent for 2 bathroom exhausts fans?


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Originally Posted by thekctermite View Post
There is nothing in the code that prohibits interconnecting bath fans into a common or shared vent. Code's a minimum standard, and I agree that it is less desirable than individual venting.
Your jurisdiction interprets IRC M1506.1 in combination with M1507.1 as allowing the interconnection of two standard residential positive pressure bathroom exhaust ducts?

Not trying to start a fight, just surprised...
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Old 11-14-2010, 05:19 PM   #18
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Can I use one roof vent for 2 bathroom exhausts fans?


In addition to everything else, most times I see a bathroom fan's exhaust duct discharging into a powered attic ventilator (PAV) and the installation is more than a few years old, the moist exhaust has rusted out the ventilator's motor.
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Old 11-17-2010, 10:15 AM   #19
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Can I use one roof vent for 2 bathroom exhausts fans?


How big is the existing roof jack [diameter]? What size would a fan like the Fantech type suggested require? It could very well be less expensive to just install another 4" roof jack.That would simplify your wiring.
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Old 05-18-2014, 10:41 PM   #20
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Can I use one roof vent for 2 bathroom exhausts fans?


I have an old house with 3 bathrooms that need vents and since 2 are on the same floor next to each other and the third is right below them I'd like to minimize the roof or house penetrations. I know you have to watch out for back flow to meet the reg.
IRC M1507.1 Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section. M1506.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building.
but I'm curious, if I put an in-line fan on the exhaust just before the vent to make sure that the air pressure keeps the moist air going out of the house, would this meet code?

and if so, how would I wire it? I've put a diagram in to show my thoughts. They'd all share the same circuit. Red would be the original circuit, blue would be the feed after going through the fan and I'd like to rig it so that the in-line fan turned on after one of the three bathroom fans turns on but I'm not 100% sure how to wire this. Can I set up a diode in-line so that they can all three feed the in-line fan but they don't feed each other?

Thanks for the help!


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