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What is the model on the pellet stove? Sometimes you can use an adapter and connect and sometimes it would be necessary to run all new.
Sounds good. As mentioned, depending on the stove model if we can use that old vent and adapt to it.
The manufacturer on this one does not recommend using larger that 4" vent so with that and along with the old being flex,I would recommend running new 3 or 4" pipe