I have a ~1980 Fireplace Manufacturers Inc. Model 42A fireplace insert. Fireplace Manufacturers Inc of Santa Ana sold the company to DESA International in 1998, which filed for bankruptcy in 2008. They maintain a website, desatech.com, but it does not have the model 42A listed. It is a metal insert, with firebrick lining.
The flue is a round cylinder connected with the firebox. The chimney is a wood and stucco frame, covered by a 2x4 metal cap. The house is in Los Angeles.
Above the mantel there is a hole in the drywall for TV cabling. Through that hole I can feel a literal breeze, so the chimney housing must be open to the outside somehow. I did not pull away all the insulation from behind the wall, but it appears that the "chimney" does not have a wall that separates my drywall from the chimney, as my drywall above the mantel apparently forms the fourth wall of the chimney, the other three walls are on the house exterior.
Also, on opposite sides of the firebox, facing towards each other from the side (just inside the metal firebox itself), are what look like vents. I get a lot of cold air in through those as well.
I took a can of foam into the attic and sealed up any openings that I could find between the attic and the stucco chimney, but the chimney is a structure on the outside of the house, so it was not really open into the attic anyway. The sealing had no effect on the breeze through the drywall.
My questions are: (a) whether these drafts can be caused by a flue that is not sealed to the metal cap at the top of the chimney, (b) can I put a tight seal around the flue and metal chimney cap to stop this draft, and (c) will sealing the flue to the top cap have any effect on the draft on the vents inside the fireplace?