A contractor just finished installation of new HVAC system with a category 1 furnace (model: GMES800805CNAA).
He originally used a reducer to connect 4 inch exhaust to a 2 inch PVC pipe. He claimed that he has done so in the past and it worked well. After we started the new unit, we ran into some issues and eventually ended up with a completely melted PVC pipe. He replaced first few ft of the exhaust with a metal 4 inch pipe, but terminated it with a PVC 4 inch to 2 inch reducer and let it vent through the wall.
His claims that PVC melted because it is summer, and during winter pipes will be completely fine.
Is it safe to use a PVC 4 to 2 inch reducer and PVC pipe venting to the side of my house?
replacing melted pvc: 
finished product: 