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Old 08-21-2012, 03:03 PM   #1
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Can I use a T to combine bathroom vents through the roof?


I'm finally going to get around to actually venting my 2 upstairs bathrooms through the roof.

The vent/lights in each bathroom are literally about 2 feet apart and what I would like to do is only have to poke through the roof deck one time and place a T below to combine both vents.

Any major issue with this approach? If it's ok would it be better to put the T at the vent through the roof or lower and closer to the bathroom vents themselves.

Thanks for any insight!

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Old 08-22-2012, 06:27 AM   #2
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Can I use a T to combine bathroom vents through the roof?


Are you talking about plumbing vents, or exhaust fan vents?

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Old 08-22-2012, 07:05 AM   #3
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Can I use a T to combine bathroom vents through the roof?


Sorry, I'm inquiring about exhaust vents.

Thanks.
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Old 08-22-2012, 09:05 AM   #4
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Can I use a T to combine bathroom vents through the roof?


I don't think combining two bathroom exhaust vents is technically a code violation, but it isn't recommended. You would have to install a check valve (backflow preventer) in each line to keep the fan in one bathroom from exhausting back into the other one, you'd need larger fans (greater airflow capacity), and would also need to make sure the diameter of the exhaust duct is the same everywhere.

It's also a much better idea to vent the fans through a wall rather than through the roof. If the layout is right, it's easier to do, and the fewer roof penetrations, the better.
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Old 08-22-2012, 12:23 PM   #5
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Can I use a T to combine bathroom vents through the roof?


Mine are done with a T, but that was back in the 1980s so I wouldn't say it met modern code or was even advisable. There is a check valve at the T and it exits through the wall of the attic. One dumb thing they did was exit through the wall that was furthest from the bathrooms! This meant that there was plenty of damp air to condense in the uninsulated pipe. Don't do that! And insulate the vent pipe, or use insulated pipe.
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Old 08-22-2012, 05:58 PM   #6
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Can I use a T to combine bathroom vents through the roof?


I don't know if it's OK or not. But I think a Y, instead of a T, would reduce the likelihood of one fan blowing into the other bathroom. I'd put the Y right at the roof penetration.
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Old 08-22-2012, 10:41 PM   #7
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Can I use a T to combine bathroom vents through the roof?


If I was using your bathroom.....and you were in the other one.......you would know I was in the other bathroom......

Most of the 'check valves' (back flow preventers) are just a simple flap....that do not seal that well.

All it really takes is for a wondow to be open...wind blow the right direction....and if someone is useing the one bathroom...not hard for the other bathroom to get a 'dose'.

I can totally understand why you want to T (or Y) them together....only on hole in the roof.....I'm dealing with the same thing right now....both of my new vents are close enough to T in....butt.....I think it is better to be on the safe side...
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Old 08-23-2012, 10:13 AM   #8
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Can I use a T to combine bathroom vents through the roof?


My understanding is that no, you can't have two fans blow in to the same vent. But, I have seen the suggestion to have a single remote fan suck from both bathrooms.
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Old 08-24-2012, 07:52 AM   #9
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Can I use a T to combine bathroom vents through the roof?


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Originally Posted by tylernt View Post
My understanding is that no, you can't have two fans blow in to the same vent. But, I have seen the suggestion to have a single remote fan suck from both bathrooms.
That is usually how it is done in an institutional setting. Large roof exhaust will service a number of restrooms but this fan runs 24/7 and wastes alot of electricity and climate control energy. In a residental two hole set up I suppose the fan could be in a three way switch circuit but that would make for some confusion at times, switch up or down, two users at the same time, etc.

If it was me, I would just put in two independant vents.
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Old 08-24-2012, 09:09 AM   #10
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Can I use a T to combine bathroom vents through the roof?


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In a residental two hole set up I suppose the fan could be in a three way switch circuit but that would make for some confusion at times, switch up or down, two users at the same time, etc.
Will code allow the two switches to be wired in parallel? That way the fan is on when either or both switches are on, and off when both switches are off.
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Old 08-24-2012, 10:56 AM   #11
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Can I use a T to combine bathroom vents through the roof?


Most fans have a damper, its gonna work as well as any commonly available ones you can get to install in your T system, or better that Y of ratherbefishing (me 2). You'll have to get better dampers from HVAC supply house not your local big box. Its possible, but doubtful, that one exhaust could back flow and over power the other one when both are on, preventing weak one's damper from opening.
Commercial and residential codes have diff requirements. I'd check up on that connected exhaust thing. if code doesn't allow it but you do it any way, why worry about the code on 'parallel' switches? Two circuits from existing switches to the blower would do the same thing. That's just a parallel, with the connections back at the main box. You need a blower and ducting that moves as much air as all individual blowers would if they were all running at same time. Some draw backs; any time you turn on one room's exhaust, you're also exhausting the other room. if that is a concern you could use electrically operated dampers . You'll need a bigger blower (I guess in this case its a sucker) and bigger duct, at least after the blower, meaning bigger hole in roof.
I think ratherbefishing (me 2) has the best solution, say 2 4" ducts into an 8 or 10, keep the Y high and with the verticle duct as short as possible from inlets . to distance you need to be above roof line and I think you'll be okay.
Then again, as Grammaw told Gramps after they'd had 12 kids down on the farm away back when, , "I told you to make that a two holer when ya was first makin that out-house."


FORGET EVERYTHING !
I just found this while looking for something else. Just go buy something like this
http://www.lowes.com/pd_89247-14-MP1...tt=exhaust+fan

Last edited by notmrjohn; 08-24-2012 at 05:09 PM.
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Old 08-25-2012, 01:38 PM   #12
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Can I use a T to combine bathroom vents through the roof?


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Will code allow the two switches to be wired in parallel? That way the fan is on when either or both switches are on, and off when both switches are off.
The single remote fan options I have seen use a low voltage relay for the switch - no line voltage switching. Basically, each bathroom has a low voltage switch that starts the fan. The fan has an adjustable run time and shuts off automatically after you hit the timed setting or press the switch a second time. They are a little pricy, but your noise level is dramatically reduced since you can mount the fan in your attic and not right in the ceiling above the bathroom.
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Old 08-25-2012, 01:53 PM   #13
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