Exhaust fan venting

Old 02-03-20, 03:34 PM
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Exhaust fan venting

HI, I recently installed a 80cfm exhaust fan in a bathroom that never had one. I also am going to change the ancient one in the master bath for a new 100cfm fan. Currently the master one uses a rigid 4in vent reduced to a 3 inch about 3 ft in that goes to an exit in the wall.

I want to run the new vent in rigid expandable dryer ducting 4 inch. Both fans are about 6 feet from the wall with the exit to outside. I want to run both ducts to the exit and connect them with a y fitting then continue out of the attic. I want to avoid making mew penetrations in the walls or roof. Especially since it is on the second floor and I dont have a ladder to reach.

My questions are is that type of ducting ok to use? And can the ducts be ran this way. Im located in California, any help is appreciated.
Old 02-03-20, 04:03 PM
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I don't think tying the two exhausts is a good idea. Seems to me that one fan running might exhaust into the other bathroom. I don't know if it's allowed per code.
Norm201 voted this post useful.
Old 02-04-20, 07:16 AM
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CW is right. Many discussion in the forum about connecting vent and exhaust fans to one outlet. Not a good idea. Also venting through side of the house near the roof is frowned upon (moisture to close to the fascia board and roof seams (check local codes to see if it's allowed). Through the roof is proper and to code.
Old 02-04-20, 11:27 AM
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For what it's worth, I did this in a house a few years ago after pouring over code references. I made the interpretation that as long as there's a backdraft damper on each fan (which practically all fans have), it's code compliant to wye them together into one vent. I also decided the potential issue of one fan blowing into the other bathroom was less risk than a second roof penetration.

It wasn't inspected, so I can't verify my interpretation would actually hold up... but figured I'd at least share my opinion on the matter.
Old 02-04-20, 12:11 PM
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Zorfdt, That's an interesting bit of information. Funny that was never mentioned in previous post on this subject matter. Would like to here from some of our other experts on the subject,
Old 02-04-20, 01:12 PM
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Two thoughts:
Personally, I'd prefer to vent them outside separately. With that said, exhausting two or more fans into a common plenum is often done in commercial buildings, but be sure that the ducting and outlet is sized to handle both fans running at the same time.
Also, it is a good idea to use insulated ducting to avoid condensation from forming inside the ductwork.
Old 02-12-20, 07:07 PM
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I am with Zorfdt. Should not be a problem as long as the fans have a backdraft dampers.

Also use a Y connection so the air will naturally go up and out the vent.

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