The duct is sized perfect to fit the hole out of the fan. The duct runs maybe 5 feet and pushed down through the soffit. It works a lot better when I took the duct off my fan. If the duct isn't obstructed the only thing I can think of is hot air naturally flows up and the cold air is restricting its movement. When the hot air is blowing down and out might the cold air be naturally blocking the warm air from moving out ?