I have a family room built on a slab. The 8" metal duct from the basement is the farthest from the furnace. Air flow and temp. are low. A duct booster fan helped with flow but not temp. obviously. Can I run an insulated 6" duct through the 8" concrete duct and insulate the rest of the metal duct in the basement? Or would that increase temp. and reduce flow? The concrete duct runs about 12ft. to the first register and about
15ft. to the second.thanks
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