Large gap in smoke chamber?

[John Arnold]

I'm definitely no fireplace expert, and I'm already calling for this to be checked out by a pro. But I don't recall ever seeing anything like this, so...
There's this large gap, like 1 - 1.5 inches, where the arrows are pointing in the photo. This would appear to be between the smoke chamber and the back of the chimney, but I don't understand how that could exist, really. I believe this is the original fireplace in a 58 year old home. What am I seeing here?

28450

[Nick Ostrowski]

Looks like a sloppy build in the throat of the flue. Uneven corbelling with a big flat shelf and no parge coating either. I'm sure Bob Harper will weigh in at some point on this.

[Bob Harper]

You've covered most of the defects:
-exceeds maximum corbelling
-exceeds maximum corbelling in a smoke chamber
-not parged
-no full head and bed mortar joints
-smoke chamber not parged.

What I would add is the corbelling is not bonded or woven in with the rest of the masonry, thus it is unstable. The camera can fool the eye but this smoke chamber appears a bit tall. Cannot exceed the width of the fireplace opening. Must have a smooth transition from smoke chamber parging into flue. First flue tile must be fully supported 360*. The chimney was constructed with CMU block. If they are >4"nominal block, they must be fully grouted in the cells or else they do not count for the nominal 4" solid masonry units.
hth

[John Arnold]

Thanks, Bob!

[Matthew Klein]

As someone who had a fire due to a gap in the flue tile and brick, I hope that the chimney does not have any openings in the brick or joints. Hot gases find the path of least resistance, whether up the flue or around the flue and into other spaces. We were very lucky.