I have heard the cracking and popping before too, [when I was burning wood] I always thought it was the glassy layer of creosote cracking from the heat. It could be expansion/contraction of the flue liner.
Most clay-lined flues will last a century or more. But if it is an outside masonry chimney, it will be harder to keep really hot to draw well.
I assume when you said you had a fire last night with 90% of the air above the fire, it was a wood fire?? Wood will burn well on a 5" deep layer of ash, many wood stoves don't have any form of grate, and require the wood ash to be shoveled off the floor of the firebox. I think wood likes the insulating layer of ash. The oxygen joins with the hot embers of the burning wood above the wood to create flames, or if not enough oxygen is present, then the unburnt gasses go up the chimney and condense on the cool chimney surfaces, creating creosote.
I have burnt VERY dry wood, it was kept in a dry shed with a greenhouse roof, so it was almost like a kiln during the summer. And the wood still created crosote. The only way to get wood to not create creosote [from my experience] is to burn it very hot, with lots of flames, to burn up all the gasses. Otherwise, creosote will condense in the chimney.
Have you inspected the chimney since those creosote stains appeared?? I'd hate to hear of a chimney fire!!
When you make your coal fire tonight, get that 4-5" layer of wood coals again, then add a 2-3" layer of nut or stove coal, open up the ashpan door vent, close the loading door vents. Let this first layer get burning, then add a second layer , still just 2-3", you want to be able to see the red fire through the newly added coal. Let it get burning as well. Then load up the firebox to the top of the firebrick. Let this get burning for 20-30 min, this will differ with different chimney draft, and stove/furnace design. But let it burn enough that you can see the small blue flames above the fresh coal.
Then close down the air under the fire to about halfway, this is a tough thing to estimate, but you want to cut back the air enough to control the heat and rate of burn, but not too little, and have the fire go out. With most Harman spinner air controls, I think 1/2 to 3/4 of a turn out from closed should be enough.
Hopefully Mark Cap and Greg White both with Harman SF models [but not your furnace] can help with the initial air setting.
Hope this helps, Let us know how it works out.
Last edited by LsFarm
on Sun Mar 04, 2007 11:37 am, edited 1 time in total.