The length of time a 50# quantity of coal will burn has many, many variables. If in a stoker stove, it may last more than a day in warm weather, maybe even two days. It would burn just enough to keep a fire going.
In a hand load stove, a 50 # load may be only 6" deep in a large firebox, or maybe 18" deep in a small firebox. Basicly, the surface area determins heat output, and depth of coal bed determins length of burn time.
So in a small stove with say a 9"x 10" firebox, a 50# load would be about 12-16" deep, and with a very low draft setting in warm weather, this coal could last for almost a day before it is all burnt up. With a cold night, and a high draft setting, it may only last 6-8 hours.
In my big boiler, I have my firebox reduced to 15"x20", I can increase this to over 25"x20" if I need a lot of heat for very cold weather. I see about 1" of coal burnt per hour with the weather around 30*. So I get about 12 hours out of a 12" deep bed of coal.
If the weather is colder, the burn rate increases to about 1.5-2" per hour. So for me I need at least a 12" deep load for a night, and later in the season, over 20" deep for a sub-zero night.
I would try to keep your coal bed as deep as you can all the time. Once you see the bed start to burn out, I see less flame, and more 'dead' areas where the coal is not glowing red, then I would shake the fire down and add enough coal to fill the firebox. Coal likes a deep pile of fuel. It is not happy with a shallow bed of fuel.
Every stove has different characteristics. So in one stove you may see the level of the coal drop only a little in say 6 hours of burning, then drop drastically when you shake the grate. Another make of stove will have the coal ash drop through the grate as it burns, so you would see very little drop when the grate is shook.
I don't know the make or the characteristics of your stove, but I'd say just keep the firebox loaded up to the top of the firebrick lining the firebox. And keep track of how many hours results in how much coal burnt.
I can only give you generalities about your questions, but my best advice is to go ahead and start burning in your stove, and keep a few records to refer back to as the season goes on. These records will help next season as well.
Hope this helps Greg L
Last edited by LsFarm
on Sun Oct 29, 2006 4:56 pm, edited 1 time in total.