I am installing a new bathroom sink in our basement. The drain line is 1 1/4" into the wall and I plan on using a 1 1/4" trap. I am installing an air admittance valve (code in my area). The vent will be on the horizontal waste line under the sink. Do I need to use a wye or can I use a tee? I am not seeing many options for a wye in this smaller size. I would assume using a 1 1/2" trap reducing into a smaller drain line wouldn't be the best move?

  • You might be interested in a product called Magic Trap which is a 1-1/2" trap kit with an integrated vent-tee that has threads for the manufacturer's AAV. You can reduce to 1-1/4" to connect to the sink. shop.rectorseal.com/products/other-mechanical-products/… Commented Nov 11, 2020 at 13:20
  • After looking at it a bit more I think my setup wouldn't accommodate the Magic Trap. It is a tiny sink, 14 deep, so not much space under the sink. My trap will head toward the wall, connect into the drain line, pass through the wall and make a right turn with two 45s. The back side of the wall is unfinished. So I would probably need to put my vent right after the 45s when the drain runs straight. This will probably be 10-12" from the trap. Is there any issue putting the vent that far away from the trap or does it just need to be between the trap and where the drain line ends? Thanks.
    – junta
    Commented Nov 11, 2020 at 18:58
  • The AAV's vent-tee connection needs to be above the trap weir (high-water level) in order to work correctly. Usually, within 5 feet of the trap. The AAV itself should be 4" above that -- it's why the Magic Vent kit has a small riser for the AAV. These measurements will be in the installation instructions for any AAV kit. The AAV also must be accessible so you can inspect and replace it in the future. It can be inside a wall as long as there's an access panel and the AAV is protected from insulation. Oatey, Sioux Chief, and Studor all make access boxes for the type of setup you want. Commented Nov 12, 2020 at 13:54
  • When I called the manufacturer they didn't mention anything, but my drain line will run about 7ft emptying into a Saniflo toilet. Will an AAV do it's job on a drain line that length?
    – junta
    Commented Nov 13, 2020 at 17:17
  • 1
    @JeffWheeler - I wanted to make sure I am understanding this correctly for this project and the previous where an AAV was used on the utility sink in the basement. So I want to make sure that was done correctly as well. So the AAV needs to be a min. 4" above the trap. But the reason why you would want to keep the AAV close to the trap is because if you went too far downstream you would be putting in the tee for the AAV actually below the weir because of the slope of the drain pipe. So the height vertically the AAV sits, as long as it is a min. 4" above the trap, doesn't ultimately matter?
    – junta
    Commented Nov 14, 2020 at 18:45

1 Answer 1


Do I need to use a wye or can I use a tee?

A tee would be the correct option as AAVs are typically required to be mounted in a vertical orientation (with an allowable variance of a few degrees from plumb). You should purchase your AAV prior to configuring your waste piping and consult the AAV manufacturer's instructions. There are also requirements for the required height of the AAV above the horizontal piping (typical minimum is 4").

...using a 1 1/2" trap reducing into a smaller drain line...

is usually not a problem

You should consult with your AHJ on the use of an AAV as not all jurisdictions allow it.

  • It seems like there is a mix of answers online about what is the proper connection and the directions don't specifically say. I may just need to call the manufacturer.
    – junta
    Commented Nov 11, 2020 at 18:54
  • I agree that they should be above the waste line but as high as you can get it, don’t wimp out and put it at the minimum that allows gunk to build up and causes failure I put them tight agents the counter in a upright vertical or as high as I can. not horizontal (I found one that way with a bucket under it because it leaked). Using a y or a t depends on how you tap the line both are ok.
    – Ed Beal
    Commented Nov 13, 2020 at 19:51

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