I have the 6" liner fit tightly inside of a 8" square telecotta flue. No room for insulation unless I was to jam insulation down from the top between the liner in the corners of the telacotta. But this would not be neat. But what you say makes sense now to me. If I try and burn a soft/light coal fire, I'll have trouble. When I really let it rip, it'll burn real nice. Reading your post from over the w/e stands true with me too. If I let the fire begin to go dead around the edges, it is too late to refill, will only smother out the fire. It definitely takes some nursing it back with some oak blocks.
I have a accurate Ashcroft temp gauge on the stack pipe. If I keep my exhaust temp at 250 F or better, I have a real nice hot coal fire. If it settles down below 225 F., it is time to throttle up the draft.
Coalsweat, I am using a hand fired Harman SF-250. Look for it under the *pictures of stoves*. I also recently installed a hot water coil in it which is really working good, maybe too good. Check out the thread.