I may have over stated the dimensions of the firebox slightly, but it should hold 140 pounds of coal according to the manual. There is no firebrick, its a boiler so the sides of the firebox are the water containers. Made like a traditional sectional boiler, each section encloses the fire box completely and the sections are bolted together to give it depth (see the diagram at http://cemrweb.cemr.wvu.edu/~mathews/boiler/page11.jpg
). The whole thing is buttoned up in fiberglass and a nice outer jacket that barely even gets warm to the touch while the fire is raging inside. Its designed for 130k btu/hour, so with my current coal testing at 13.7k btu, that would be about 10 pounds per hour, or 60 pounds for 6 hours??
If I could get that length of burn I'd be a happy camper, but the box seems to just go dead unless I keep getting out there and poking it every couple hours.
Berlin, do you know what your chimney temps should run at normally? I'm thinking it could be a draft problem, although if it is I don't know what steps would make it better.
The chimney I made is 6 inch steel pipe, a 45 out of the boiler, up 2 feet, a 45 to get to vertical and up 2 more feet, then a 90 into insulated double wall for 2 feet, a last 90 back vertical again and then up for 6 feet. I had a chimney roof on top but removed that thinking it could be causing soot buildup.
If its lack of draft, a fan forcing air into the bottom should help, but it didin't seem to make much difference.