This is a 4 Pellet Pellet Wall Thimble with the Air Intake, this is from the PelletVent Pro Line. It has the duralock tightening system, to get a tight fit. Use this to bring in fresh air from the outside for combustion within the wall thimble, so no secondary hole needs to be cut. This comes with an insulation ring to reduce air infiltration where the pipe goes through the thimble. It is a 3 diameter and the 18 flex hose length will expand to 5 . Clamps are included.