PelletVent Pro 4″ Pellet Chimney Wall Thimble This 4 Inch Pellet Chimney Wall Thimble from the PelletVent Pro series is a Type L Biofuel Chimney Pipe component and consists of the duralock system for a tight seal. Use for through-the-wall installations. Fits between standard wall studs. Comes with an insulation ring to reduce air infiltration where the pipe slips through the thimble. Features: Type: Wall Thimble. Fits Between Standard Wall Studs. Type L Biofuel Chimney Pipe Component. Specifications: Overall Dimensions: 9″ (W) x 8″ (D) x 9″ (H).