Code in your area calls for a 2" pipe, and what is installed is a 1.5" pipe, is that what you are saying? I'll just say that it used to be Code to have a 1.5" diameter drain for washing machines, and these drains still work today. Where I live it still is Code. I had a top-loading washer for 20 years on a 1.5" drain. Now I have a front-loading washer. My building has 50 more drains like this; it's a highrise condo building. I think people will tell you that a 2" drain handles the flow better, and doesn't risk overflowing. FYI a front loading washer uses much less water than a top loading washer.Originally Posted by UtahSpike
Also, regarding 1.5" or 2" drains, Code used to call for a 1.5" drain pipe for showers and now it calls for 2" drain pipe. Many people who install new showers onto old 1.5" pipe reduce the new shower drain to 1.5" instead of replacing the whole pipe and Tee. What a larger pipe does is carry more water and air, the air being necessary to make up the volume displaced by the moving water. Note that Code calls for a 1.5" drain pipe for a tub-shower. I think the rationale is that a blockage in that drain pipe is less critical there since the tub would hold the water that backed up.
Best draw a sketch for your other, bigger, questions.