Vent reduction allowed?

[Kenneth Rivalsi]

I am redoing my basement bathroom. Toilet, shower and sink. I was planning on venting each of the 3 fixtures each their own 2" vertical vent and then combining (above the flood level rim) into a single 2" vent.
I just now realize that the existing vent (which combines with the main vent in the attic), is 1 1/2".
Is it okay to REDUCE the vent for basement bathroom from 2" to 1.5"?
I would do the reduction in a vertical orientation.

thanks in advance, Ken

 

[Paulypfunk]

No. In the UPC code minimum size venting for a toilet is 2”.

You may consider using a Studor vent or AAV to supplement for the natural venting shortfall.