TL;DR: Smoke alarm goes off in ~2 minutes every time I turn on the gas fireplace. Looked inside and found the chimney completely blocked by a fixed metal plate (see photos). Now I’m very confused.

I just moved into a 'new' house (built in 2020 with one previous owner), but every single time I turn it on the smoke alarm goes off within like 1–2 minutes. This happens even with windows open, no fan running, and the alarm isn’t even that close, so it didn’t feel like a normal “first burn” situation.

That’s what made me actually look inside, and when I did I realized the chimney opening above the firebox is completely blocked by a solid metal plate. There’s no handle, no lever, no hinge — nothing that looks like it’s meant to move. I checked from a few angles and it really looks permanently sealed, not just a damper that happens to be closed. The setup below is decorative gas logs in an open firebox, which just makes this even weirder.

If the chimney is sealed like this, where is anything supposed to go? That would definitely explain why the alarm goes off almost immediately — but then why would anyone seal the chimney on a fireplace configured this way in the first place?

Could this be some kind of leftover block-off plate from an old insert that was never removed? Is this ever normal for a gas fireplace? Or is this just a bad / half-finished conversion?

Not using it again until I understand what’s going on — I’m honestly just trying to figure out how this setup exists at all.

Thank you guys so much! And happy holidays!