Our home was built in 1980 and had a built in fireplace. The last couple winter we have been getting some smoke in the room. Decided this morning I was going to find out what was going on. Started taking it apart and found a metal tag with mfg information on the side panel I removed. Found out it is a Preway Builtin Fireplace from Sears, mfg. in 1980.
After removing the right side panel I found a 6" door the leads, indirectly, from the fire box to the room blower surround. A slide control rod at the bottom front of the fireplace controls this door. Either lets it op or holds it shut. Believe there is one on both sides. I had no idea what the control knob was for or what it was supposed to do. Made sure the control was operating correctly and put it all back together and cleaned up my mess. Will find out next time we built a fir in the fireplace if that was the problem but an sure it was. We had been running with that control in the open position, which would draw air from the firebox and blow it out into the room.
Someone else was having a similar problem and this might steer them in a fix direction.