Well, I've got to move the water heater in the basement to make some more room. The chimney & footer is in the center of the house, the water heater is on one side & I want to move it to the other side. I can get the wet core machine from work & core appx 12" through the chimney into the flue—is there any reason why I can't/shouldn't do that? I can also raise the breaching somewhat, right now I've only got about 8" from the top of the draft diverter to the breaching.
The inside dia of the flue is ≈ 7½ x 11½. I'm sure this was adequate for the old gravity furnace, but I'm down to just a water heater in that flue now, 33k input. I think this is a good time to think about a flue liner as well. Height to chimney cap is ±30', straight shot. Is there any reason to not install a 3" or 4" insulated flue liner since I'm already going to be doing some major rework? The flue seemed to be in fine shape the last time I was on the roof, maybe two years ago, several years into the current config (just the WH in the flue).