First, Id open up that crown and see if a liner could be dropped down to the left of the fireplace. In my area, a chimney like that would be hollow inside.
Your Biasi boiler works good as a direct vent. You would need the Biasi direct vent kit to do this. This would use the burner to exhaust everything, not a powerventer.
The liner would probably be cheaper however, if possible.
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