Have a Goodman Furnace that continues to fail to start up. After some diagnosis on my part I find the incoming vent PVC pipe is clogged with frost. I believe the incoming air is drawing in moisture from the hot air exiting the furnace and then building up frost just inside the pipe opening. I can snake the line and clear the frost and system works fine.

My intake and exhaust pipes are side by side where they exit the house. How far apart should the two pipes be from each other?

The furnace should have been installed exactly as the manufacturer stated in there instruction manual that comes with every furnace from the factory. Exhaust and intake lines are specifically addressed for different units and different manufactures.

Frost can be a problem if the pipes are located close together since the exhaust is discharging moisture laden vapors that can be drawn into the intake. Follow the manufactures install instructions to the letter and it should work with no moisture problems. NOTE if your clothes dryer is also close that can also cause a problem.