2" mip and bush it down to a barb fitting. clear vinyl tube, loop it to create a trap, then head it to the drain.
that thing is nothing special.
I'm guessing your working with a Hiel or tempstar

edit to add. I would be more concerned about the riveted heat exchanger.
any roll out in the burner chamber or yellowing of exhaust pipe (pvc)

I believe some of the NULK's had that issue.......BTW.......those heat exchangers have a life time warranty.
Good Luck.