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Old 10-07-2009, 02:03 PM   #1
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two 3" bathroom ducts to one 4" exhaust?


I can't seem to find any charts today that tell me what I need to know about duct capacity, so I have a quick question.

I need to finish replacing rusted out roof vents today, and the bathroom exhaust fans have me slightly stumped. There were originally two 3" flexible metal ducts from the two bath fans poked up inside a 9" slant-back vent. That vent is being replaced with a 4" gooseneck, but I'm unsure if I can wye both 3" ducts in to a single 4", or if I need to separate them completely and add a second 4" gooseneck. Both fans have 3" outlets and flaps.

Memory tells me that a 4" should work for exhausting both 3" fans, even if they run at the same time, but I've been having more and more of those senior moments these days.

So, do I need to hit the hardware store and get another 4" gooseneck to toss up there, or can I just put off the trip to the store for now, and tie both of the exhaust ducts in to the single 4" gooseneck with a wye?

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Old 10-07-2009, 02:32 PM   #2
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two 3" bathroom ducts to one 4" exhaust?


2 separate vents. A 4" won't carry enough air for 2-3" to wye into it.

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Old 10-07-2009, 02:52 PM   #3
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two 3" bathroom ducts to one 4" exhaust?


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2 separate vents. A 4" won't carry enough air for 2-3" to wye into it.
That's the safe bet, either way. I was just hoping to avoid making another hole in a roof full of brittle asphalt shingles. These things like to crack if you look at them for too long.

Thanks for the input!
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Old 10-07-2009, 03:21 PM   #4
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two 3" bathroom ducts to one 4" exhaust?


Uhh... Just climbed back down from the roof. I thought I might be able to use a 4" roof jack where the slant-back vent had been, but I can't. There simple isn't enough roof deck to nail the vent down to because of the size of the hole that's there. Whoops.

I really don't want to get in to messing with the deck at this point, because it will turn in to a can of worms. Can I use an 8" gooseneck up there and tie the 3" vents to it? The only point I can come up with against this is the size of the flap in such a large vent and the added weight it has. I have this mental image of it slapping open and shut as air pressure rises and falls in the ductwork when only one exhaust fan is in use.
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Old 10-07-2009, 04:16 PM   #5
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two 3" bathroom ducts to one 4" exhaust?


An 8" would be large enough.
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Old 10-07-2009, 05:01 PM   #6
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two 3" bathroom ducts to one 4" exhaust?


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An 8" would be large enough.
Good enough for me. Home Depot stocks those. If the flap won't lift, I'll just take it off. Haven't had a vent with a draft seal on the roof since 1965, so it can't hurt much to not have one now. Thanks, beenthere.
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Old 10-08-2009, 06:55 AM   #7
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two 3" bathroom ducts to one 4" exhaust?


Wow. This is awesome. I was under the impression that you couldn't connect 2 bath exhaust vents together at all.
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Old 10-08-2009, 11:10 AM   #8
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two 3" bathroom ducts to one 4" exhaust?


Not uncommon for 2 or more bathrooms to be exhausted to the same HRV/ERV, along with the kitchen.
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Old 10-08-2009, 03:05 PM   #9
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two 3" bathroom ducts to one 4" exhaust?


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Wow. This is awesome. I was under the impression that you couldn't connect 2 bath exhaust vents together at all.
In some places, you can't. For that matter, it's apparently not allowed here, either. It's always best to consult your local codes so that you know for sure. The thing is, it was allowed when my house was built, and I simply needed to replace an existing leaking roof vent, so I can get away with leaving them both exhausting through a single roof penetration because they've always been that way.

Don't get me wrong, I'd much rather have two separate roof jacks, but doing that would leave me with a giant hole in the roof deck to contend with, and I don't have the time and money to deal with that right now. I'll correct the deck and separate the vents when I replace the roof, which will probably be next Summer. For now, I just wanted to stop the water intrusion that I was getting from having vents with rust holes in them.

For anyone wondering, the exhaust fans work great, now. I guess that's what happens you you remove a few decades worth of old wasp nests from them.

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