Fireplace question

I just completed the InterNachi course on Fireplaces, Chimney’s etc. Afterwards, I inspected my own fireplace - and I was a little unsure of a few things.
Firstly, the fireplace has a metal insert within the firebox, inside of a masonry fireplace. I believe these are called Heatform boxes, built in when the fireplace was constructed. It has a ‘heatilator’ type of setup as well, that heats air from the room and blows it back out.
We had the fireplace WETT inspected and swept last year, and had a new operable chimney cap installed. However, the fireplace has not worked properly for us. Smoke frequently comes back into the home. One of the points that I noticed is that there does not appear to be a damper where it would usually be, it appears that it may have been removed at some point. Is this normal? Or, could this be the causeo of the issue of smoke coming back into the home? Also, there seemed to be some form of insulation present at the top of the firebox within a gap area. (photo attached)
Since we had it inspected I had assumed that everything was up to code. However, I’m a bit stumped as to why we still have the issue of the smoke returning into the home - and why there appeared to be some insulation at the front of the firebox.
Thanks for your help!

Has the fireplace always had problems drawing? Have you ever tried to crack a window to see if the fireplace starts to draw properly?

We’ve only used it since it was repaired a few years back, maybe half a dozen times or so. We have tried cracking a window - but it’s a bit hard to say whether that really helped, since we usually only open it once smoke has come into the room. The fireplace does also have a fresh air supply from outside and glass doors, so in theory it should draw up properly even with the doors closed

Yes, we’ve tried opening a window. I think that it did help it a bit, but it makes the room cold - which kind of defeats the purpose of having a fire. The unit does have an exterior air supply and glass doors too

Agree. Home may have a negative pressure preventing a proper draw… can you say “back-draft”?

have you tried starting the fire with the exterior vent blocked and again with it open and got the same results? is the top of your chimney above the roof line? Im surprised it passed a WETT inspection with out a damper.

Years ago we had a fire place that to start a sheet of news paper up the chimney light it and away it goes ,Draft started then it worked great .

If it has a fresh air intake it could be blocked with a bee’s nest or even a mouse nest. I would check that as well.

This may help a bit.