I've read this thread with great interest, as well as the older "minor explosions" thread, since once I got going with my coal fire, I discovered booms!....I am a newbie; finally got my fire up and running last January with the excellent help from you all on this forum!
I found the booms very disconcerting and have had the chimney clean out door blow open. Booms are something I really wanted to eliminate.....but how? I think I finally discovered the system for my stove, a Hitzer 55.
The stove has a number of good features. The internal baffle that improves combustion efficiency has a big 6" diameter hole in it, which is opened and closed by an external control. It is designed to allow a direct path from firebox to chimney to stop smoke coming into the room when you open the loading door. I find the direct path afforded ideal to usher out the volitile gases produced from new coal.
There is a spinner draft knob on the loading door to allow over fire air.
The fire is controlled by a thermostat that mechanically opens and closes the primary air damper feeding air under the grates. It has worked very well and controls my fire within a 20F range. I run my stove around 400F surface temp.
Keeping in mind the advice on this forum to never cover a fire with fresh coal, only tend a lively fire and watch for the blue ladies....I would tend as follows:
Open my direct vent damper, spin open the spinner draft knob in the loading door and open the ash door and wait. In just minutes my fire would come to life and reach 550F, sort of an arbitrary temperature I settled on, as it provided a very lively fire that I could shake and poke and not hurt it in the least!
On hitting the 550F point, I shake and then stoke one side. The temp will drop with the fresh coal and then recover to 550F again at which time I open the loading door, see lots of blue ladies on the new coal on side one, and I stoke side two. Again the temp drops with the new coal and recovers to the 550F point...normally a 10 to 15 min process. Now I am fully stoked, the fire has recovered to 550F and hopefully the volitiles have burned off. I close the ash door.
This is when I'd get booms.....a lot of time. Not bad, but booms that are disconcerting! to say the least. I've got air going over the fire as the spinner is still wide open. I have the direct vent damper open so it's a straight shot up the chimney. I had a vibrant fire with loads of dancing ladies at the 550F point when I closed the ash door......so why the booms? ?
You folks keep saying you need flames to prevent booms! Ok....made me think. When I close the ash door, maybe I am shutting so much air off....as in ALL the air coming under the fire....that all my blue ladies go out....setting up booms. No way for me to watch....no glass door in my Hitzer.
And OK, I could crank open my thermostat to open the primary draft control and let air come under the fire....but then I'd have to remember to set it back to where I run it....yeah, I'm lazy and like it real easy.
A small nail to the rescue! Yep, I decided to try propping the primary draft door open just a crack. A 6 penny finish nail worked like a charm. That lets enough air in under the fire after the ash door is closed, to maintain some blue ladies and keep burning the volitiles. Yet the damper is closed "enough" to bring the fire down to operating temperature. Takes awhile, as in maybe 30 minutes, but I've not had a boom since going to this technique. Once I close my ash door with the nail propping the primary damper open slightly, I can go about my business as the stove is safe....not the most efficient with the direct vent still oen and spinner still open, but safe to leave if I forget it. (I never leave the stove while the ash door is open.)
I'll set a timer to remember to return and improve my stove efficiency by closing the direct vent and door spinner in about a half hour, at which point I'll find my stove at operating temp (400F) and the auto damper has opened slightly to control the fire, which drops the nail out onto the floor and the control is again capable of closing the air off tightly, if need be, but long after the volitiles are burnt off.
I know this doesn't help folks with different stoves but I will stress again to follow the sound advice on this forum. To avoid booms, you must have blue ladies the entire time it takes for fresh coal to give up its volitiles. A direct path out the chimney is a big help. Over fire air is good.....but doesn't make up for no blue ladies if the under fire air is cut off or insufficient.
I think we all understand never to completely cover a fire a fresh coal....even when in a hurry....it's been said you must have patience to tend a coal fire....I found that to be VERY true. Stoking in two steps isn't that bad. My entire process from open the ash door to close the ash door is about 30-40 minutes. Add another 30 minutes to come back and close the spinner and direct vent for efficiency...but I can be away from the stove for that time, doing something else.
My two cents....