I have an existing chimney that runs vertically through the middle of my house. This house was built in 1890. I do have this grate or vent made of what feels and sounds like cast iron, that is covered up from the inside with a metal plate. I was able to remove this metal grate to see what was behind, and I found very old brick and what looks like a mini firebox. The floor of this mini box is made of very thick slate, and does extend outward, under the existing floor in the living room. The back of this mini firebox is slanted inward a bit, just like a normal size firebox. I was wondering what this used to be, and if I would be able to make this a normal sized fireplace using the existing chimney. Just a Note. My furnace does not vent out this chimney as it is gas, and vents out the side of my house. However, I am not sure if the water heater vents out this chimney. Your input would be greatly appreciated. I have a picture of the iron grate that is covering this chimney.