I have a whirlpool AFG 8271 frost free freezer, above the top most shelf (marked pizza) is what appears to be a plastic vent. From this water appears to come out and freezes, it sometimes looks like a mini frozen water fall when I open the door.When I open the food trays there is frozen pools of water on the lids of food boxes.The temp always appears to be a static -18. Can you please assist me and tell me what's wrong.

Tanks in anticipation

Colin Learmouth