Lennox Furnace

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Is the ductwork going underneath a crawlspace to the addition. If so then it should be insulated. 6 or 8" flex works well. Any professional will agree to performing a heat loss calculation for the total square footage that the furnace is heating. This will give you a proper btu input rating, so the furnace is not over or undersized. The take-offs you mentioned should be coming off the main supply trunk with at least 6". If not then maybe you can install dampers and adjust the air flow. As far as price goes you, it would be next to impossible to give you an accurate quote without seeing the job.