Anyone know what this is?

[Marc M]

It doesnt appear to be connected into the main fireplace anywhere, no holes, except that big one in the middle.

[Rick Hurst]

Not to any crawlspace I'm guessing or you would have crawled down into it right?

[Jerry Peck]

Marc,

Where were you standing, in the attic looking down? In a basement looking down?

[Dennis Eckmann]

If I am looking at the picture right and you are looking down from the fire box then it is a ash pit they used them in coal fireplaces there are normally a clean out on the floor below I have a house that has 2 of these one on second with a cleanout on the main floor and the one on the main level has the clean out in the basement

[Marc M]

Oops, I should have said where I was, sorry. I was in the attic looking down at the fireplace where the it comes thru the ceiling.

[Bob Sullivan]

You may be looking at a "dummy" flue built to create some sort of aesthetic effect below the ceiling line in the structure's interior. It's not properly lined as would be a functional flue.

In terms of thermal performance, this "hole" may be an enormorous energy leak if all that separates the conditioned space of the structure from the attice is a single wythe of brick. If that's the case, suggest plugging this "hole" with insulation.

[Marc M]

Yea, interresting... I thought it was an after thought or mistake.

[Mike Truss Guy]

You guys are so informative. I was going to say it was a hole in the floor. :D

[Robert Dalga]

Marc,
It may also serve as a fresh-air intake into the FP??? Did you see an openings/vents on the insides of the firebox? Is that insulation that is down inside the opening? Hopefully not.