I replaced parents old unit with Goodman 70,000 BTU 80 furnace it is condinsating.I have raised blower speed it does no good.(no other unit or water tank is on stack just new unit)Pipe goes from 4in 5in to 7in in stack.Do I need B pipe and do I still need a cleanout?I was told four inch pipe will be sufficient?

All ways use what the manufacture states. I'm sure venting with 7" pipe is not good. That's your point of moister build up.