While working on something in my crawl space today,I noticed this and it didn't seem quite right to me.
The duct on the right is from the furnace. What makes me wonder is the way it bends down before splitting off to the registers. The low spot isn't on the ground, but isn't off the ground by much. I checked the duct supports and they don't appear to have slipped or otherwise malfunctioned. Is there some reason they installed the ductwork with a low spot? Is this unusual? Is it something that needs to be corrected?