gap between flue and firestop

[Bob Brooks]

I have two fireplaces which are stacked between floors between interior walls. After opening a electrical outlet found a excessive amount of cool air coming out. After further investigation found this opening was into what I would call the flue box. After opening the box at the attic floor found there was a gap between the flue and the fire stop plate of approximately 3/8 of an inch in spots.
Can I put a fire stop product around the flue and the fire stop to fill this crack?
I was told by the fireplace manufacturer that applying a product directly to the flue would cause a hotspot at that point on the flue and deteriorate the flue.

[Jim Luttrall]

You got the right answer from the manufacturer. As unlikely as it sounds, the gap is there for a reason and is tested as manufactured. To try to "fix" it will violate the listing and might cause other problems.

[Bob Brooks]

Thanks for your quick feedback. is there any other solution, this gap is causing me two problems cold air being pulled down into my house between interior walls. The other problem is when there is no wind this cold air brings the smoke from the fireplace down that flue box into the walls and out into the room.
Wouldn't a opening such as this allow a fireto migrate pass the fire stop?
is there a way you could explain to a novice what would happen if I closed this gap with a fire stop.
If your answer to the above is something I would live with what type of caulking, rating, brand would you recommend.
Thanks again

[Jim Luttrall]

I was told by the fireplace manufacturer that applying a product directly to the flue would cause a hotspot at that point on the flue and deteriorate the flue.

Re-read what you provided earlier.
This gap at the firestop is not the cause of your problems, look elsewhere and fix the real problem. Better yet, get a fireplace specialist to exaimine the installation. There is no way this issue is causing smoke to reenter the room. You need someone who understands the entire system, not answers from an online form. We cannot see the installation and any help from us will be incomplete.