Newbie Main Drain Question

[rbknox]

Hi, I am remodeling two bathrooms side by side. A simple diagram attached for crawl space drain lines - in horizontal plane. I want to replace 4" cast iron drains with 3" ABS (at large X) and also move hall bath wc to wall with lav. The problem is the connection into main line is 2" for lav and location is at junction of other branch. I would like to get to proposed configuration with 3" ABS. However changing 2" drain to 3" could get messy if scenario A as it seems I would need to replace all cast iron combo fittings. Ideally scenario B keeps things simple (all upstream of X), but not sure if this is even allowed (drain flows back and then forward in horizontal). Or is scenario C the way to go. Also attaching pic of circled junction to show complexity. Looking for advice on best way to proceed. Thanks.

 

[rbknox]

Hi, I am remodeling two bathrooms side by side. A simple diagram attached for crawl space drain lines - in horizontal plane. I want to replace 4" cast iron drains with 3" ABS (at large X) and also move hall bath wc to wall with lav. The problem is the connection into main line is 2" for lav and location is at junction of other branch. I would like to get to proposed configuration with 3" ABS. However changing 2" drain to 3" could get messy if scenario A as it seems I would need to replace all cast iron combo fittings. Ideally scenario B keeps things simple (all upstream of X), but not sure if this is even allowed (drain flows back and then forward in horizontal). Or is scenario C the way to go. Also attaching pic of circled junction to show complexity. Looking for advice on best way to proceed. Thanks.
View attachment 94217View attachment 94218