I have an American Standard single stage freedom 90 gas furnace. It has a power vent, allowing me to vent horizontally.

I want to know what size vent pipe is required. It goes about 40 ft, with a 90 degree elbow in it.

There is 2" pipe now, which I found out was too small for my water heater. So, while I'm at it, I want to make sure I have the correct size for the furnace.