I see this string was posted back in 2012 but I’ve got the same problem now in 2018 with my Rheem. I thought it was a vent issue because I had a low clearance. I switched the PVC from 2 inch to 3 inch diameter and used a long sweep 90 elbow. There is no venting issue now but the blower temp and the exhaust temp are really hot. The PVC is hot to the touch and so is the blower and I’m wondering, based on the 6 causes you printed (#5) if that is the issue. If the blower/exhaust temp are too high, then what are the causes/fixes? The Rheem people sent me a new blower in case the blower was faulty and I installed that and no change. It blows fine but the exhaust is really hot.