Amana furnace that was installed in my house used 2" pvc for the exhaust. i was told by a technician that was out for a repair that it should have been vented with 3" pvc. Model#gvva070ax40

How long and how many elbows. If you have the manuals, the install manual will tell you what sives, lengths and elbows allowable. I do see this problem quite a often.