I am installing a pedistal sink in a modular home, (bathroom). The old sink was cabinet style and the drain pipe went down through the floor. I need to move the drain pipe back about 4" so it is hidden inside the wall.

My question is that there is that the current drain pipe has a vent on top. The vent is only 8inches higher than where the T comes out to the sink drain. (1) Is this vent really necessary? (2) if so, should I extend it up higher than the 8 inches since it's going to be inside a wall? (or can I just take out the T and put in a 90, eliminating the vent all together?

can I just take out the T and put in a 90, eliminating the vent all together? You're asking me if you can build a "S" trap and the answer is NO you can't.

) Is this vent really necessary? Every code will tell you: "Every fixture that has a trap MUST have a vent".

should I extend it up higher than the 8 inches since it's going to be inside a wall?You don't have to extend it but code mandates that all AAV's be out where they can be serviced.
Install the AAV just after the trap as shown. Good luck, Tom

Thank you. That's going to be uglier than home made dirt. Most drains are in the wall, (not straight through the floor). Why haven't I ever seen that vent sticking up like a sore thumb before. It seems like the pipe just goes straight into the wall.

Oh well,

Thanks

Tom is correct. You need a vent. His suggestion is the easiest sollution. You can also run the aav up inside the wall and put an access hole to the aav behind the mirror. This would make it accessible, thus making it code approved. The only other way to do this correctly would be to run a vent all the way up through the roof. Since you will be cutting into the wall anyway and will need to do patchwork, it wouldn't be that much more work to run the vent up and out. Good luck and please let us know what you decide. Lee.

Thanks. I thought about the roof option, but it's a vaulted ceiling with a block at 8'. I was only planning on patching drywall down low since we will be covering it with wainscoting board afterwards. I think the service panel behind the mirror is the best idea. My biggest concern was if the pipe got clogged water could potentially get backed up and coem out through the vent. Under a sink wouldn't be bad - but inside a wall would really bite.

Thanks guys. Now that I know it has to be vented and the vent has to be accessible - I'll figure something out.

Let us know if we can be of further assistance. Good luck, Tom

Just one more thing, if the drain does plug up, it will back up and overflow the bathroom sink before it will come out the vent(as long as the vent is higher than the rim of the sink). And yep, vaulted ceilings can be a bearcat to get a vent up and out. Please let us know how you make out. Lee

Just one more thing, if the drain does plug up, it will back up and overflow the bathroom sink before it will come out the vent(as long as the vent is higher than the rim of the sink). And yep, vaulted ceilings can be a bearcat to get a vent up and out. Please let us know how you make out. Lee

Lee,
Take a good look at a AAV,(see image)
1) You can't instaill a AAV higher then the flood rim if it's mounted just after the trap in the cabinet.
2)You are correct that the fixture will overflow before the AAV but not for the reason you gave . It won't back up out of the AAV because it's a one way fitting. It will let air in to vent but once it vents it closes off just like a check valve. Regards, Tom

I agree Tom, but what I'm suggesting is not placing just after the trap, but at the san tee in the wall(on top of tee). In this case you can take the aav a high as you like, it will still draw in air. Also, this way if aav fails(not common but does happen) water will not be able to get up to aav and leak. Another way to picture it(and I would never do this, but have seen it done). Is to picture a correct drain vent set up for a bathroom sink, only instead of running a true vent out through the roof, you install an aav in the attic. Like I said, I would never do this, nor recommend it(as in my area the aav would freeze due to humidity in vent). Im not sure we are looking at this the same way, but I willing to see it your way, just show me the way. Lee

But Lee,

this way if aav fails(not common but does happen) water will not be able to get up to aav and leak.
You're still overlooking the fact that the AAV can't leak if there's a back-up plus I've never had one to fail yet.. If he runs it up the wall he will have to install a access panel in the wall and that's going to look funky. Much better to hide it in the cabinet. Regards, Tom

We are definitely on the same page. Access panel would look bad, but I mentioned in post #4 that he could make the access behind the vanity mirror(of course I realize this would mean the mirror would have to be easily removeable, or if using a medicine cabinet, you would have to look at that access cover every time you opened the door. I truly do agree with you Tom, but I'm just brainstorming to find ways he won't have to see the aav on a pedistal lav. This is a tricky one. Of course you know that my code doesn't even allow aavs, so I may not be the right person to answer this question, but I'm just trying to give some possible options. I definetely agree that if it was a vanity cabinet, I would put the aav in the cabinet, but they are installing a pedestal lav, so that's why I'm giving the crazy options that I'm giving. Please let me know what you think. Lee

Of course you know that my code doesn't even allow aavs, so I may not be the right person to answer this question
Not quite so Lee. I researched your code and guess what?

Under the Uniform Plumbing Code (UPC) ALL VENTS must terminate through the roof, however the UPC will allow a maximum of one AAV per structure, however, The UPC only allows a maximum of one AAV per structure with the expressed consent of the local inspector . I agree that a AAV looks a lot better ina cabinet the hanging on a pedestal so I'm going to go with your plan for installing the AAV on a raiser in the attic. Nice save Lee! Of course I'm assuming the bathroom ceiling isn't vaulted.

Regards, Tom

Nice find in the UPC Tom. I missed that one. My local inspector always said I couldn't use them, so I just assumed I couldnt(which I guess I can't since he won't give me his consent.
Now that I think about it, in may last cec class, the instructor did mention something about the one aav per structure. I believe he said it was a good idea because it would save having to island vent island sinks. And I agree. I actually think an aav would be better and much easier than an island vent. We are soon to get a new inspector, so we will see what he has to say on the subject.

The asker did mention vaulted ceiling, but I also haven't seen to many in a bathroom. Hope asker will come back and clarify. Thanks again Tom for the input on upc aav. See you later. Lee.