As some of you know I've played with burning a lump or two of coal every now and then just to see how it does.  I finally called up a buddy who works at a coal dock and he said to come by and we'd load my truck down.  I took my truck up and hand loaded it completely full of softball to footlball size lumps.  I heaped it up and tried to place it to maximize my load.  When I left my ball was probably 5-6" off the ground haha.  
Sooo..   for the last 3-4 days I've been playing with the coal and will continue to experiment and let you guys in on what I'm doing.  
I'm heating 22-2300 sq foot double wide in a Natures Comfort 175 furnace.  The past several days have been typical winter weather, a lil warmer, but nothing crazy.  Lows 10-15 Highs 25-30.  Typically my burn times on seasoned hardwood can range from the shortest burns of 14 hour or so in zero weather to as much as 24 hours when the temps only bottom out in the high 20's.  Since I started burning a bit of coal, I'll go out at night and throw in probably 20-30 pound of coal.   Then add about 1/2 a load of wood over the coal, where as before I would cram it full.  My burn times are now easily over 24 hours.  I'll fill at 5 or 6 pm, go back out around 9 am open the door and smile.  There will still be wood in there and those chunks of coal will be laying there glowing red.  I'll usually just punch it around and wait for my evening fill.  
This evening after a 24 hour burn I am sure that my stove would have went another 8-12 hours.  I threw in about 20 pound of coal, and about 1/3 a load of wood.  I'll update on how this done.  Tonigt it is supposed to be about 10-12  degrees.