
The OD of the narrow end is 2 1/4 inches, which is the size of a large Shop Vac hose. Therefore, to use this, you need a screed. In fact, the large end's OD is the same size as a 4-inch hard-galvanized duct, so you'll need a coupler on that end as well, unless you're connecting it to a dehumidifier's flexible vent hose. I bought this and then had to go to the hardware store to buy two more pieces to make it work! :(

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