I plan to install a toilet, wash basin, and a laundry tub in our basement. Will break out the floor and hook into 4" DWV line. The run from the main line to the toilet is about 5 1/2' feet. I figure that if I vent at the point where the new toilet drain line enters the 4" main line, I should be okay. Basin and tub will be directly above the main line. My question is about the vent pipe. According to the information I can find from on-line articles, it seems that a toilet draws 4 "fixture units", the basin draws 1 and the laundry tub 2. 7 altogether.
There is a single laundry drain pipe nearby that drains into another drain pipe (3") which then runs directly into the vertical stack next to the basement wall (can't use it for my purposes). The vertical stack runs directly into the 4" line under the basement floor. That 2" laundry drain is vented by a 1 1/2" pipe that goes to the roof. Also in that same small system is the kitchen sink (with dishwasher and disposal). The kitchen sink and laundry drains work excellently. The vent pipe is actually 2" where the kitchen drain hooks into it, but then it converts to 1 1/2 " as it turns to go through the floor upstairs. If only that were a 2" vent stack (which should be able to handle 16 fixture units..more than enough for everything)!
Am I risking too much by hooking the vent line for the new half bath and laundry tub into that 1 1/2" line? Will it do the job? It doesn't seem to pass code, but would it if I asked for an exception? I certainly can't run a vent pipe from the basement to the roof. I can get to a 2" vent line that is currently only serving a toilet and washbasin...but it will be very difficult to get to it (have to tear out some finished areas, including first floor vanity and wall, etc., and will have a very tight working area to get to the vertical stack above that first floor sink. Need some expert advice on this. Thanks.