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Old 07-22-2009, 01:10 PM   #1
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2 Bathroom fans to 1 roof vent?


I am wondering if I can run two exaust fans from two separate bathrooms to only one roof vent with a T or a Y. Currently the main bath upstairs has a bath fan that vents through the roof, the master bath has only a window and i'd like to install a fan near the shower (but not in it) to provide a little extra light and to exaust the warm moist air when the shower is running. The roof is fairly new (I think 2 years old) and I'd like to not put another hole in the roof. The two bathrooms are back to back on the upper level so the vent lines would be fairly short. I already have to replace the main bath fan and in my recent remodel, I ran the wiring for the master bath fan up into the attic, planning on installing the fan later.

I know the fans have a damper on the output opening preventing backflow of air, and it's unlikely that both fans will be running at the same time.

Will this work? And if so do I need to be concerned about CFM output of each bath fan in the event they do run at the same time? What would be the best way to tie both fans into the same roof vent? "T" or can I find a "Y" with the correct fittings?

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Old 07-22-2009, 04:16 PM   #2
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2 Bathroom fans to 1 roof vent?


Y connections are used all the time in new construction to connect bath fan vents. Shouldn't be a problem. I wouldn't worry about CFM...Even good fans don't move a heck of a lot of air. Length of the vent and number of bends is much more critical to good performance.

Be aware that the code requires that the fans vent to outdoors. The inspector may or may not accept a roof vent as an acceptable termination point.

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Old 07-22-2009, 10:50 PM   #3
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2 Bathroom fans to 1 roof vent?


Well when I say "roof vent" I mean it is a round hole cut specifically for the main bath fan, not like a square attic vent. The main bath already had the fan installed as it does not have any windows or anything. The master bath has a window, but it doesn't seem to work too well keeping the mirror de-fogged and it takes a long time to exhaust the hot moist air. And in the winter time it'd be better to have the fan than opening the window, there is a heat vent at the bottom of the bathroom vanity.

Good to know I should be ok with two fans on one external vent. I already have several feet of the 4" insulated vent line that I used for the basement bath exhaust fan; around here the code required it be insulated. No big deal, found a box of 25' of it and only needed about 7-8 feet for the basement bath (if even that much).
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Old 07-23-2009, 08:34 AM   #4
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2 Bathroom fans to 1 roof vent?


Most builders use a metal Y connector to make the connection in the attic.
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Old 07-23-2009, 12:41 PM   #5
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2 Bathroom fans to 1 roof vent?


Maybe Termite can clear this up. I thought that when two powered vent fans used a common exhaust outlet that they had to have backflow dampers to prevent one fan from exhausting back through the other. Some fans have them built in, but I'm curious if there is a more stringent requirement in this case.
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Old 07-23-2009, 01:22 PM   #6
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2 Bathroom fans to 1 roof vent?


I'd think, at a minimum the pipe that carries both outputs should be twice the cross-sectional area of each fan's pipe.
A = PI x dē/4
With parties and guests both fans may be running at the same time.

As to sizing this link may help.
http://www.engineeringtoolbox.com/ve...tems-t_37.html
but you kinda' need to know the "fan curves". If the curves are "lying on its side S" shaped rather than "lying on its side L" shaped you need to run the fans in stable regions.

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Old 07-23-2009, 02:20 PM   #7
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2 Bathroom fans to 1 roof vent?


My understanding has always been that 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."
prohibits the interconnection of typical positive pressure residential bathroom ventilation systems as this can result in recirculation of exhaust within the structure when when one or more fans are active and the other(s) not, while it's OK to connect two or more sides of a negative pressure system when the extractor fan is located beyond the junction(s) (ex: a typical multi-inlet Fantech installation).
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Old 07-23-2009, 06:28 PM   #8
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2 Bathroom fans to 1 roof vent?


I know the new fans I will be getting for both bathrooms has the back flow damper on the output exhaust of the fan. Its the same fan I installed in my downstairs bathroom about 2 months ago.
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Old 07-24-2009, 07:54 AM   #9
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2 Bathroom fans to 1 roof vent?


Quote:
Originally Posted by Michael Thomas View Post
My understanding has always been that 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."

prohibits the interconnection of typical positive pressure residential bathroom ventilation systems as this can result in recirculation of exhaust within the structure when when one or more fans are active and the other(s) not, while it's OK to connect two or more sides of a negative pressure system when the extractor fan is located beyond the junction(s) (ex: a typical multi-inlet Fantech installation).
the code doesn't look like it's prohibiting an inter-connection, only prohibiting recirculation of the air. You can't exhaust your bathroom air into the basement or the garage. Two fans on one vent (properly sized) would still vent to the outside. In my mind the question is about the size of the vent and the location of, and need for back flow dampers.
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Old 07-25-2009, 03:17 AM   #10
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2 Bathroom fans to 1 roof vent?


With the backflow dampers I can't see how the air could be re-circulated in the house. I don't know how large the roof vent is, though I assume its a standard 4" vent.
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Old 07-25-2009, 05:43 AM   #11
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2 Bathroom fans to 1 roof vent?


The dampers on typical residential positive pressure bathroom exhaust fans are designed to prevent gross backdrafting of cold external air back into the system, and are not designed or intended to produce the level of airtight sealing required to completely prevent backdrafting. The only question is how much backdrafting you allow before you are violating the non-recirculation requirements.

Ultimately, this is a call for municipal code officials, but I can tell you that at home inspections you are frequently able to feel the backdrafting at an inactive exhaust fan in one bathroom when the exhaust fan in another is active and there is a cross connection in the positive pressure portion of the vent system.

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Old 07-28-2009, 01:42 PM   #12
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2 Bathroom fans to 1 roof vent?


I think we all recognize that the ideal installation doesn't involve interconnecting the vents of bath fans. That being said, the stated code language does not expressly prohibit it in any way.

Two violations that I've encountered illustrate what the code is trying to combat. Both circumstances involve hooking the bath fans up to the HVAC system. One was a cold air intake on a home's HVAC system that the contractor attached the bath fans into (because it leads to outside)...Failed. The other is when someone hooks the bath fan vent to the home's return air system...Also failed. Another obvious thing is the code doesn't want the vent going into the attic where water vapor can freeze, settle, mold, etc.

We're not talking combustion gas vents or something like that. The backflow dampers in the fans don't need to be military grade hardware to be generally effective. The worst thing that happens is that a small amount of humidity (or the smells associated with Mexican buffet dinners) gets introduced into another room if it bypasses the damper. No big hazard there.
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Old 07-28-2009, 02:25 PM   #13
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2 Bathroom fans to 1 roof vent?


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Mexican buffet dinners) gets introduced into another room if it bypasses the damper. No big hazard there.
But methane is explosive!
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Old 12-15-2013, 08:25 AM   #14
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2 Bathroom fans to 1 roof vent?


I just came across this posting, it's best to check all local codes. When connecting two exhaust fans to one exterior vent its best to remember a couple of important things. Make sure the exterior vent can handle the cfm.
Use rigid pipe, this is better to ensure no back pressure or restriction.
Use a damper, flapper this let's air out and restricts the back flow into the home.
Keep vent away from exterior window.
Do not vent into attic space, moisture from the bathroom to a cold attic is a breeding ground for mold.
Also in New Jersey I heard its against code to use the flex venting for a cloths dryer. 4" ridged line with fittings make it easier to clean.

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