My friend Sarah redid her basement and covered the old floor vents with laminate, then had the worst smoke smell every time she used the fireplace. It was so bad they couldn't even stay in the room. Her dad came over and told her she probably blocked the air supply, so they cut out a small piece of the new flooring near the hearth. Sure enough, there was a little metal vent grate under there. Once they cleared it, the smoke went right up like it should. She felt so silly for not checking before.