I had a basement wall stubbed out for a future wetbar. I expected the hot and cold water pipes and the sewer pipe coming out perpendicular to the wall which is the case. However there is what appears another sewer pipe extension coming out diagonally above. Any idea what this is designed for? Do I need to use it at all for the wetbar sink or can I cut it back closer to the wall and plug the end? If this allow me to I will attach a picture.

Can you attach a picture and show us? Back to you, tom

Picture would be best, of course, but I'm betting that pipe could be for you to attach a mechanical vent... also called an AAV (air admittance valve). See if you can find a vent associated with this wet bar plumbing... if not, then you need to install the AAV under the cabinet for future access should it fail to work.

Back to you...

MARK

Just a guess but I think that the diagonal pipe was installed there for future use as a clean out.

Agree. Diagonal makes me think someone used a wye for a cleanout.

Hi,
Thanks for your replies. Was not sure how to upload the image. Hopefully this will work. Note that there is already a cleanout at the bottom of the wall, so this diagonal extension is obviously for some other purpose.

It must be intended for you to install a mechanical vent as I previously guessed... ;)... lucky guess, for sure!

Here, you'll install an 1.5" pvc 45 degree fitting to straighten out the pipe. Then you'll install an 1.5" female (threaded) adapter and then screw in an AAV (air admittance valve). Be sure to use some teflon tape on the threads of the AAV. This AAV will act to vent your sink... see image below.

Most AAVs are installed like in the bottom picture, but since your pipes were brought up in the wall the plumber had to stub that pipe out of the wall so you can install the AAV in an accessible spot under the sink. AAVs can fail so you may need to replace it in the future... you'll know when it fails because you will smell a sewer smell when you open the cabinet.

MARK

Can the AAV be installed inside the wall? I thought it had to be outside as the diagonal pipe for this person would allow.

Another question. On an existing install (sadly, mine) where the vent pipe is 12' feet or so downline, could an AAV be installed on the extension from the p trap into the drain?

No... an AAV cannot be installed inside the wall... why I said that the AAV needs to be installed under the cabinet in an accessible location so it can be changed in the future (read my last post).

12' is too far like you thought, so you would install an AAV exactly like in the pictures below.

Let me know if you still have questions on this... O.K?

MARK

Can the AAV be installed inside the wall? I thought it had to be outside as the diagonal pipe for this person would allow. AAVs must be installed where it can be serviced. The askers rough has the AAV under the bar sink not in the wall.

Another question. On an existing install (sadly, mine) where the vent pipe is 12' feet or so downline, could an AAV be installed on the extension from the p trap into the drain?

Where would the vents connection be? At the trap or 12' downstream from it? Regards, Tom

SB, I'm looking at the diagram above. It shows a small pic at the bottom right with an AAV on the extension which runs from the p trap to the wall drain. My sink drains OK but could be a lot better. That would seem to be a better answer than having to get into the wall through kitchen cabinets. Does that make sense? Is it ever done that way?

If this is the picture you're looking at, (see image) That's where the AAv should go. If you're asking if you can run a vent line 12 feet from the trao and install a AAV on the terminal end, the answers , yes.
The AAV can go any place as long as it takes off near the trap. How near depends on the size of the pipe. It all depends on where the take off point is and I'm not clear on where yours is. Can you enlighten me?
Regards, tom

I was thinking of exactly what is pictured. Come off the trap, go two or three inches, install the AAV on the extension before it joins the drain at the wall.

Massplumber, thanks very much for your help! The illustration is great!

I was thinking of exactly what is pictured. Come off the trap, go two or three inches, install the AAV on the extension before it joins the drain at the wall.

Works for me! Good luck, Tom

Thanks. That sure simplifies things.

You can now add me to the list of people that believs that the diagonal pipe is for an automatic vent.

Hi all:

Godle, glad if the drawing helped!

Mark