Venting question


Old 03-20-09, 11:41 AM
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Venting question

Hello. I've trolled for awhile and enjoy reading these forums as a source of information. I have a specific question so I've come out of the woodwork.

Just wondering on the best way to tie venting into the main stack in my scenario. I have an AAV in the basement that I have the opportunity to remove and tie that vent line into the stack and vent properly. Looking for opinions on right way.


In the diagram, the middle 1-1/2" vent line in both Figure A and B is the one coming up from the basement. The 3" line is just vent at this point - no water is draining above the vent tie in - no wet venting.

Figure A was my first thought, but wasn't sure if it was acceptable and what the size of the horizontal line (branch vent?) should be. My original thought was it should be bumped up to 2", but wondering if 1-1/2" might be acceptable.

Figure B was my second thought, but for a variety of reasons is not my preferred choice.
Old 03-20-09, 01:33 PM
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fig. A could work with the horizontal piece in question being 2" but it depends on what code your area follows and what else is connected to the 3" and what other vents out the roof you may have if any
Old 03-20-09, 04:24 PM
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FrostedGlass: One key piece of info. you didn't include was what are the 1 1/2 vents connected to?
Old 03-20-09, 06:47 PM
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Sorry, I tried to keep it simple. Apparently I kept it too simple. Hopefully the revisions help (I did not indicate the p-trap on the bathroom vanity - it is there).

Revised Diagram

The 3" going out the roof is the only one for the house. By way of background - small bungalow, mid 1950s construction with single bathroom. DWV system is cast iron with copper drain pipes and galvanized vent pipes that join the cast via threaded connections which is why I prefer Figure A to limit the "cuts" into the 3" cast iron soil drain/vent stack.

There is one other vent line connecting to the 3" higher up than shown in the original diagram. This is existing and is 1-1/2" dia venting the kitchen sink drain.

The 1-1/2" vents come from a bathtub and basement sink. The basement sink is the one that is currently has an AAV that I'm trying to bring up and connect into the main vent stack.

Not shown on the diagram is the toilet that connects to the 3" vent/soil stack and the 1-1/2" drain from the bathtub. This is the same bathtub that the 1-1/2" vent mentioned above is for. The bathroom drain set (vanity, toilet, bathtub) I prefer to leave as is if possible. The only proposed change to this system is to the bathtub vent.

I hope this helps and makes sense. I guess my underlying question is can I have the short branch vent at 2" or am I better off with connecting them separately to the 3".

I'm in Ontario and so am, as I understand, under the Ontario revisions to the Cdn NPC here.
Old 03-20-09, 08:02 PM
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Well, unfortunately I cannot speak for Canadian code but here in U.S. either fig. A or B are fine as long as they new vent ties in at least 6" above the flood level rim of the highest fixture served on that vent. An 1 1/2" vent is plenty for what you are doing.

But once again you must check with your local codes for exact answers.

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