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Old 04-04-2012, 01:17 PM   #1
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Bathroom exhaust fan ducting


I am currently installing a new bathroom in my basement. I would like to install an exhaust fan venting to the outside. Above this basement is my upstairs bathroom which currently does not have an exhaust fan. I would like to install a fan in this bathroom at the same time. My question is regarding the ducting. Can I run the 3" ducting from both of these fans into one pipe to outside? My foundation is quite thick and I currently have a 3" hole to the outside. I am trying to avoid drilling another hole. If this is OK then what type of pipe and connections will I require? Thanks.

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Old 04-04-2012, 01:25 PM   #2
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Bathroom exhaust fan ducting


You can but those two three inch pipes will have to be increased to something like a single five or six inch. That's going to have to be the size of the hole in your wall.

So it'd be a 3 x 3 x 5 or 6, the last number being the final exhaust size. It's called a Y.

Btw, you measure the bathrooom individually length x width x hiegth and divide by 8, the amount of air changes per hour, to get the correct size cfm fan for each restroom. Also, "sone" is the noise level of the fan. You can pay a higher premium to get a lower "sone" (quieter) fan.

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Old 04-04-2012, 01:49 PM   #3
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Bathroom exhaust fan ducting


Thanks for the information. Somebody mentioned to me that if you had 2 fans run into the 1 pipe that the exhaust may back into the other bathroom when one of the fans is used? Enlarging the hole to 5" would still mean renting the hammer drill so I may go with the separate vents. Would you use galvanized pipe or plastic?
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Old 04-04-2012, 01:57 PM   #4
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Bathroom exhaust fan ducting


Neither but if I had to pick one, pvc.

We use a flexible aluminum duct. It's pictured in this pic and notice what happens when it's not vented properly to the outside or to the soffit.

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Old 04-04-2012, 06:00 PM   #5
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Bathroom exhaust fan ducting


They sell backdraft dampers to prevent back drafting back into the other fan. You would need one on each line prior to the wye. But finding them in 3 inch is hard, I've tried. Most everything is 4 inch now.

But as mentioned, in order to use both fans at once, you really need a larger duct past the wye.

So it might be best to run separate ducts. HD sells a Milwaukee 4 3/8 inch carbide grit hole saw that should cut trough most anything and the hole will fit most 4 inch duct terminations. You will need an arbor to go with the hole saw. About $40 for both I think. Then you can use widely available 4 inch duct terminations with a 3 to 4 inch adapter.

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Old 04-05-2012, 10:23 AM   #6
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Bathroom exhaust fan ducting


Thanks, I can use 4" ducting then with the backdraft damper. This will work for me but how large does the outside duct need to be? Thanks everyone for the info.
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Old 04-05-2012, 03:09 PM   #7
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Bathroom exhaust fan ducting


Here are the areas for each duct cross section (pi * r squared):

3" = 7.07"
4" = 12.57"
5" = 19.64"
6" = 28.27"

I'm not a HVAC contractor, but I'd say you need at least a 5" duct to flow as well as two 3" ducts. And the 5" has to be part of the wye, because the smallest duct is going to control all the flow. So you can't add on a 3" to 5" expander after the wye, because you are trying to cram two 3" flows through a 3" wye outlet and then trying to open it up. You can't use 4" duct and a 3" duct termination for the same reason.

If you only tend to use one fan at a time, or you can live with some reduced flow when both are in use, you can probably get away with using 4" duct. Probably a lot easier to find 4" duct and 4" terminations. But if you are going to open up your outside wall to install one 4" termination, you may as well do two 4" terminations and give each fan its own outlet.


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