Using existing exhaust stack for new furnace
You cannot use the flue in it's current state. High efficiency furnaces exhaust less heat out of your house. The exhaust gas is cool enough that it condensates that's why it needs a drain and also why the exhaust pipe is PVC The condensate is slightly acidic and will corrode even some types of stainless steel overtime. You can however, use the old flue as a chase to run your PVC pipe through. It's not easy but if your house has the furnace located in the middle in a finished basement, it's sometimes the only option.