Washer box/utility sink issue

[kevin1877]

I recently installed a washer box so that my washing machine does not drain directly into the utility sink. The old setup was just a 1 1/2" cast iron pipe tapped into the main stack. I replaced the metal pipe with 1 1/2" PVC. Starting from the far left I installed an AAV on a vertical run of about 1 1/2' of 1 1/2" PVC going to a sanitary T. Then I put the washer box on a 2' run of 2" PVC reduced to 1 1/2" just above the P-trap. Next comes the utility sink and then I tied into a threaded connection leading to the drain stack. Everything seems to be working fine except that I get some backflow into the utility sink. Is this just because of the 1 1/2" pipe and the volume of water or did I goof up the installation?

[MG]

Should be a 2" pipe minimum. Washing machines pump out water at a fairly good rate.

[kevin1877]

That's what I figured but the 1 1/2" pipe was already tapped into the main and I wasn't interested in breaking up the floor to make the change to 2". Other than the bit of backflow into the slop sink will this cause any problems?