Should I Add an Air Admittance Valve?

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Jim the dude

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Apologies for the bad drawing. I laid the pvc for waste lines in this bath and realized that (maybe?) I should add an air admittance valve somewhere. The old bathroom vented fine, but I've changed the floor plan somewhat, hence the question.

Essentially, it is a 10'x6' 2nd floor bathroom in a duplex (halves are mirrors). Vertical 4" cast iron stack in the center wall with a 1 1'/2" copper vent going up through the attic and through the roof.

Should that vent be sufficient? I don't know if I should add an AAV near the tub drain or in-between the tub and commode.
 

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Jeff H Young

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I don't know your code but I think I'd continue with the 4 inch through roof
 

wwhitney

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If drainage is from the left to the right in the diagram, and the only vent is at the sink, then yes, neither the tub nor the WC is properly vented. The tub should get a dry vent, which can wet vent the WC. An AAV is one option, or you could run a dry vent up the left hand wall and across the ceiling to join the sink vent.

Cheers, Wayne
 

Jim the dude

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I dont know your code but I think Id continue with the 4 inch through roof
I should clarify. The 4" stack is cast and goes up to the 2nd floor, where it has a 'T' or a bell, or a hub, depending on how it's described. Each half of the duplex has a section of 4" cast running horizontally into it. The 1 1/2" copper vent comes directly out of the top of the hub, and runs vertically through the attic and roof.
 

Jim the dude

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If drainage is from the left to the right in the diagram, and the only vent is at the sink, then yes, neither the tub nor the WC is properly vented. The tub should get a dry vent, which can wet vent the WC. An AAV is one option, or you could run a dry vent up the left hand wall and across the ceiling to join the sink vent.

Cheers, Wayne

Thanks for the info. The existing copper vent isn't off of just the sink, it actually sits on top of the main (off of the 4" cast) and served to vent the entire (old) bathroom. The sink has its own piece of 1 1/2" pvc that runs into a 'T' (3-3-1 1/2) just before the main.
 

wwhitney

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Thanks for the info. The existing copper vent isn't off of just the sink, it actually sits on top of the main (off of the 4" cast) and served to vent the entire (old) bathroom. The sink has its own piece of 1 1/2" pvc that runs into a 'T' (3-3-1 1/2) just before the main.
I'm having trouble following your terminology, but the sink should have a trap, and a trap arm that connects to your stack, with at most one pipe diameter of elevation difference on the trap arm between the trap outlet and the stack connection. Then the stack above that sink connection is your sink dry vent, assuming nothing else drains into the stack above that connection.

That arrangement is what I was basing my comment on. The top of the stack dry vents the sink, but not the tub. If the tub drain connected directly to the stack, with just the lav above it, and again with at most one pipe diameter of fall from the tub trap to the stack, then the stack could wet vent the tub. But the tub is too far away for that, so it needs its own dry vent.

Cheers, Wayne
 

Jim the dude

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I'm having trouble following your terminology, but the sink should have a trap, and a trap arm that connects to your stack, with at most one pipe diameter of elevation difference on the trap arm between the trap outlet and the stack connection. Then the stack above that sink connection is your sink dry vent, assuming nothing else drains into the stack above that connection.

That arrangement is what I was basing my comment on. The top of the stack dry vents the sink, but not the tub. If the tub drain connected directly to the stack, with just the lav above it, and again with at most one pipe diameter of fall from the tub trap to the stack, then the stack could wet vent the tub. But the tub is too far away for that, so it needs its own dry vent.

Cheers, Wayne
Wayne,
Thanks again for your thoughts. I apologize for any confusion. The sink will have its own p-trap.
I don't think a single 1 1/2 vent out the roof meets your code
I looked at the air vent again and it is actually a 2" copper pipe, which is 1/2 the size of the 4" main, so it does meet code. With all due respect, having a 4" pipe going up through the roof for a vent not only seems like complete overkill (and I don't think I've ever actually seen one that size going through a roof to vent air) but I think would look ridiculous. Again, imho.
 

John Gayewski

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2" is not half of the size of 4".

The formula for area is 3.14xR ²

Most roofs have a 3 or 4 inch vent. 2 inch for a single kitchen or something, but along with that you'll find more vent for the rest of the plumbing.
 

John Gayewski

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Jim my friend I think you need more help than you realize with this project. A floor plan with pictures or a picture with your current plan drawn over it is sometimes the ticket.
 

Jim the dude

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Jim my friend I think you need more help than you realize with this project. A floor plan with pictures or a picture with your current plan drawn over it is sometimes the ticket.
I appreciate the thoughts. I remodeled the other bath 5 years ago (the one that is a mirror of this one), and everything was, and is, fine.
When I did the other bath, the commode fell perfectly between 2 joists and I ran an AAV off of the pipe between the tub and the commode. This time around, there is a floor joist right at the 12" rough in mark, so I need to use an angled toilet flange, which makes it impossible to put an AAV in the same (mirrored) place as the other bath, hence my wondering where it could go otherwise.
Truthfully, both baths have existed for decades with the single (evidently too small) air vent that is still there, which again, services the shared main stack. It is the only stack in the whole house. The current kitchen was placed over a crawl space and if there is any type of air vent added, I don't see it.
Concerning the mathematical formula to determine the size pipe that is necessary. I am wrong. I said the stack is 4" and the vent is 2". My math is wrong according to your formula yes, I was going by this statement on finehomebuilding (dot) com: Vents are required to be at least half the diameter of the drainpipe that they serve, so a 3-in. drainpipe serving a single-family home needs a main vent that is at least 1-1/2 in. dia.
Using that info, my math isn't accurate but the idea that a 4in pipe can use a 2in vent seems to be copacetic.
Again, not to be argumentative, but I'm sure my thoughts here can be picked apart ad nauseum, and I evidently, can't get my thoughts across correctly. So sorry.
 

John Gayewski

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If your building drain is 4" then you need half of the crossectional area vented. Which means at some point you'll need that. One 4" pipe being vented by a 2 inch pipe isn't the issue. We size by fixture units and 2 can vent a lot you just need more than that to vent your whole duplex.
 
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