Its rusting because the flue gases are recondensing before they leave the chimney. You should be able to run sheet metal flue pipe to a wall penetration, but I would check the installation instruction. To the best of my knowledge you can't run an 80% furnace through pvc pipe. The temperature of the gases are too hot. The reason that you can do this with a 90% is that they have a second heat exchanger that extracts more heat before it enters the flue pipe(this is where it gets the added efficiency).