stand (queitly) beside your fan as it is blowing air into the stove, when the fire is roaring all it can listen close and see if you here it puffing or speeding up and slowing down. this is a sign of very quick back drafting where the stove is trying to pull air in and then acutall trying to exaust air out. if you hear this then open teh furnace door a tad and everything calms down....your fire is not getting enough air! this can happen if you have too much overly dry wood in there or if you have too many small pieces of wood with many edges exposed to burn. remember wood does not burn, it must change to gas first...heat changes it to gas then if the heat is good, the air is just right..poof you get fire. with too many small pieces in there, they are turning to gas faster than the air for combustion is being fed to it, so the gas goes out the chimney as smoke... we can alikley attest to opening the door too quickly and a rush of air is supplied and poof..singed eyebrows