Rinnai Gas Direct Vented Space Heaters all ship with a standard Flue Kit-FOT-151 that can accept wall thickness of 4-1/2-Inch to 9-1/2-Inch. If you have a thinner wall thickness-then you must or use a different Flue Kit such as the FOT-150.
Extends From/To-3-Inch to 4-1/2-Inch
Hole Diameter-3 1/8-Inch
Typical Application-Thin Walls Mobile Homes