Can I tie two bath exhaust fan vents together?

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  #1  
Old 11-02-10, 11:05 AM
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Question Can I tie two bath exhaust fan vents together?

I have two adjacent bathrooms, one of which has an exhaust fan that vents through the roof. I plan to add a fan to the other bathroom, but want to avoid drilling a hole in my roof. So, is it OK to tie in the two exhaust lines into one vent that goes out through the roof? I'm planning to use a Y connector for it. Note that there are backflow dampers right next to each fan.
 
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Old 11-02-10, 02:55 PM
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No, you may not connect two fan discharges to a single roof jack (the proper name for the roof outlet). Depending on the size of the roof jack you could use an in-line fan with the suction side split between the two bathrooms.
 
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Old 11-02-10, 03:07 PM
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Hey furd...to clarify...is it "may not" or "should not"?

Also...what if its a roof mounted fan outlet? You know..the flat ones with the flap...is it still a jack?

These are more for me than the OP.....
 
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Old 11-02-10, 03:34 PM
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IF the two exhaust fans were electrically wired so that only one of them could be run at any one time then a single roof jack could be used although it would still be poor practice.

Any roof (or wall) penetration for ventilation purposes is called a jack, regardless of its shape or the inclusion or omission of a back-draft damper.
 
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Old 11-02-10, 03:47 PM
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Hi Furd,

Is backdraft the only issue? What if i put in backdraft dampers on each end of a Y?

The plan I have is roughly like this:

Roof Jack?
| |
| |
| |
| |
| \
| \ Y connector
| |\ \
|A| \B\
| | \ \
| | \ \
| | \ \
| | \ \
| | \ \
| | \ \
| | \ \
| | \ \
| | \ `----------------- Fan2
| | \ D
| | `-----------------
| |
| |
| `-------------- Fan1
\ D
`--------------

Will this work?

Ciao
 
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Old 11-02-10, 04:10 PM
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Additional backdraft dampers will only add to the discharge duct resistance and THAT will cause the performance of the fan to drop significantly. It could also cause a fan motor overload which will decrease the life of the motor significantly. These same caveats are true when the discharge duct is decreased in area (diameter) from the discharge connection of the fan or if excessive lengths of discharge duct is used.
 
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Old 11-02-10, 04:17 PM
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I'd like to weigh in again....
Installing another outlet for a fan really only takes about 10 min or so on the roof...less if you've done it before. And if installed properly, are no less likely to leak than the shingles themselves.

When I moved here..there were two bath exhaust fans AND the dryer vented to one T style roof jack.....just crazy!

Took me about an hour on the roof to make 2 new vents for the bath fans AND replacing the range hood vent.

Firm believer in individual vents for each fan.

Even with the large quiet inline fans..you are sucking conditioned air out of rooms where it isn't needed.
 
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Old 11-02-10, 04:45 PM
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I understand the concerns, guys. Unfortunately I have never gone up on the roof, let alone dug a hole through it. I do not have the tools and I'm afraid I'll screw it up. Not to mention it will take me way longer. With the rains and everything, I really don't want to be in a position where I might have to repair a roof leak :-( Hence, my preference to avoid a new hole in the roof. I have replaced a bath exhaust vent before (actually upgraded the fan and replaced the 3" duct with a 4", with backdraft damper). And I have cut through drywall, done electrical installation and also duct work. So I'm confident with those.

Would it makes things easier if I get rid of the existing two dampers and just keep the two new ones? Also, are dampers better if they are in a section of a vent which is horizontal instead of vertical?

Thanks so much for your inputs.

-- Ciao
 
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Old 11-02-10, 06:14 PM
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I don't blame you for not wanting to go on the roof, in my younger days roofs didn't bother me at all but these days I will do almost anything to avoid it.

All decent bathroom fans have backdraft dampers in their outlet so an additional damper serves no purpose but to increase the duct resistance. Roof jacks are available with or without backdraft dampers. If your roof jack was six-inch diameter with four-inch diameter ducts from the bath fans I would say it was okay, but not preferred. If you do connect both fans to the single roof jack be sure to use a Y fitting and not a tee.

Unless you are on a really limited budget you might drive around your neighborhood and see if someone is getting a roof replaced. If you can find one, talk to the job foreman and ask if he or one of his workers would like to make $50 on the side installing a roof jack for you. It would be worth it to avoid having to crawl up on a roof in my opinion. You could also do the Y fitting now and keep your eyes open for a re-roof job in the spring and summer and get it done correctly.
 
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Old 11-02-10, 06:40 PM
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Hi Furd,

Excellent suggestions. I'll try them out. I have to go up the attic to find out if the roof jack is 6" or not. And, yes, I always intend to use a Y and not a T.

BTW, do you know if dampers work better when they are in a horizontal section versus a vertical one?

Thanks to you and Gunguy45 for your help and suggestions in this regard. Hope to get this project working soon.

Ciao
 
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Old 11-02-10, 11:26 PM
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Dampers, with the hinge on top and mounted in a horizontal duct will take far less pressure to open than will a damper in a vertical duct.
 
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