Washer Drain Overflow

[JDCam]

I am having a problem with my washing machine overflowing during the first few moments of the drain cycle.

Apart from not having a trap (seperate problem) either the vent is inadequite or the pipe diameter is not enough. Due to the cost and difficulty of repairs in this area, I am trying to avoid guesswork. Any comments would be appreciated.

here is what I have:
- There is an in-wall plastic drain box that connects to 2" pipe.
- after about 3" the 2" pipe reduces to 1.5" pipe and travels down approx 30" to a 1.5" Y
- Upstream from the Y (12 inches) the pipe is capped off
- Downstream from the Y (30 inches) is another Y creating a 1.5" vertical vent pipe.
- another foot downstream the pipe enlarges to 2" with an elbow and disapears through the concrete.
- The main 3" stack is approx 7' from there


I am considering...
- Replace the 1.5" verticle pipe from the drain box to the first Y with 2". But from here the horizontal travel would still remain 1.5"
- or, move the vent to branch directly off the verticle 1.5" pipe

Any advice would be appreciated.

thanks
Cam

[JDCam]

Here is a crude sketch of what I am talking about

Thanks,
Cam

[jnaas2]

The 2" pipe should not be reduced in size all the way to the washer machine box. The 1.5" vent line is fine but there should be a p-trap installed before the line reaches the vent connection to keep sewer gas out of your house

[LazyPup]

I am attaching two illustrations.

The first one is your drawing with the code violations annotated

The second is an easy solution for you.