Running OAK up chimney

Okay, I've had my pellet insert three years now, and decided to run a OAK up the chimney. It's been drawing air from the room since it's was installed, but I'm tired of the draft in the house. The stove has a two inch inlet for air. What is allowed material to use. It has to go 15-20 feet. Should I increase it to a three inch? After all, it's only drawing outside air. Anyone have any thoughts?
thanks

Only non-combustible tubing should be used. If you're going to have that length, then I'd use 3" for sure.

I was thinking of using metal EMT conduit. Besides it not being insulated, would there be any other issues? At $20 for ten feet, it seem more reasonable than running some type of flex.