When you have lost the fire, was there any portion of the grate not covered by coal?? The only problem with banking a fire is that the shallow part of the firebox without the deep new coal will burn out sooner, leaving a way for combustion air to get around the pile of coal. This greatly reduces the air to the active coal bed, and soon it will go out.
So if you have deep coal on one side of the firebox, once it has started burning you need to either add more coal to the shallow portion, or rake the coal level.
I don't bank my fires. I have a deep firebox, and if I have a hot bed of coals, I add 4-6" of nut and stove mix coal, give it half an hour and it is a bright red with 6"+ blue-white flames, then I add on another 6-10". This appears to 'smother' the fire, but you can hear the coal cracking and popping, and 30 minutes later it is all glowing red again, and it will burn for 10-12 hours with a 14-16" deep coal bed.
I would open your manual damper at least 20%. This will increase your draft. What I suspect is happening, is that when your fire is fresh, you have enough heat in the chimney that even with the manual damper closed you have enough draft to keep the fire going. But as the coal burns down, you have less heat, therfore less draft and soon your fire goes out. Try leaving the damper open more, and close off the combustion air a little to compensate for the added draft.
You may need to get a barometric damper to even out your draft.
Greg L
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