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Bertha 02-22-2010 11:41 AM

Bath fans--how to send 2 to 1 exhaust outlet
Hello all,
we're in the middle of a remodel and I have 2 areas where, if possible, legal, and feasible I'd like to exhaust 2- 4" bath fans to 1 4" wall outlet. In the master bath I have a water closet/toilet room and main bath area w/shower and sinks. I'll have a bath fan in both areas, (1 @ 50cfm and the other at 110+cfm, both w/4" exhaust). Is it possible to send both into some sort of Y and then out 1- 4" wall outlet. Is it possible, ok w/building code, efficient. I realize that 1 may exhaust into the other, would inline damper be effective. Another item, I'm assuming that only one bath fan of the 2 are being used at a time.
The other similar situation is with a small 1/2 bath and laundry room setup, with an exhaust fan in each room, again both going to 1- 4" outlet.
thanks for feedback

jogr 02-22-2010 01:06 PM

I wouldn't have two fans pushing to a y. High likelyhood of odors and humidity pushing past the flapper into the other room. You might be able to get one fan that will pull from a y so that when the switch is on in either room the fan pulls from both rooms.

AndrewF 02-22-2010 04:17 PM

I wouldn't do it.

I'd vent each one separately to the outside.

HooKooDooKu 02-22-2010 04:20 PM

I don't know enough about codes to sight a specific reference that would or would not allow such an instillation. But here's a few things about code I do know to consider.

1. Mechanical codes require that all exhaust ventalate in a manor that doesn't create a nusance. IMHO, if one bathroom exhause is partially venting to another bathroom, that's a nusance. So any dampers used would need to be air tight.

2. Codes require that manufacturer's instructions be followed. So unless the instruction manual includes directions for combining exhause, a building inspector could turn it down for not following manufacturer's instructions (and the few instructions I've seen for exhaust fans always show a dedicated vent).


Originally Posted by Bertha (Post 404105)
Another item, I'm assuming that only one bath fan of the 2 are being used at a time.

Why can you make this assuption. Are you going to make it impossible for both bathrooms to be used at the same time? It would seem that if the manufacturer requires a 4" exhaust, that if you are going to combine two exhaust lines into one, you would need to also upsize the exhaust at the "Y" to 6" (a 6" exhaust is more than twice the size of a 4").

beenthere 02-22-2010 05:06 PM


Another item, I'm assuming that only one bath fan of the 2 are being used at a time.
Kind of like saying you don't need 2 bathrooms then. So just close off one. Then you only need one fan.

While it may not happen often that both fans run at the same time.
It will happen.

hvac benny 02-22-2010 06:05 PM

I would use an inline fan for this. The have only seen two bath fans connected to a wye once, and every time one would be turned on, the damper in the other would slam open. I would also reconsider using 4" pipe as it would most likely make the fans very loud.

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