I want to convert my wood burning fireplace to a natural gas insert. I've had two reputable companies come out and bid on the job. My chimney has some significant structural damage, and both suggested a partial rebuild of the chimney stack and crown (around $7k). That it is in addition to the cost of the conversion. I can't get a straight answer for why this is necessary. Couldn't an insert just vent directly outside and then seal off the chimney? Any advice?