It makes more sense in my situation to tie in a 2" to my current 4" (which comes through the roof) in the crawlspace.
So, in the basement, this new 2" will be used for the sewage pump, the basement bathroom group, and the kitchen sink vents.
Does that sound correct?
I'm still a bit unsure why the kitchen sink cant tie into the upstairs vents even though I am about 2" inches above the rim instead of 6" and what the danger is. Is it just that, if the bath sink clogs, the water will come through the vent to the kitchen sink? I dont see a differnce between 2" and 6" in that situation. I mean, is there a difference between 6" and 2" in terms of what the code accomplishes?
In any case, if I can safely use all three of them on my new 2" then that is what I will do.
Does that make sense?