Just saw this post, and since I've been burning coal more successfully this year in my Yukon Eagle, I thought I would post some info for anyone who may have one of these. I'm no expert, but I hope my experiences help others out there! There is also some more info in this thread as well: Yukon Husky Multifuel Furnace
Well, I found out as some others did that the air inlets don't really get under the coal very well, so I ended up drilling five 5/16" holes in my ash pan door. With the air inlet built into the stove closed, there is still enough air to pull into the bottom of the fire to keep it going. Speaking of the built-in air inlet, it is controlled by a solenoid with a mechanical linkage to a butterfly door. Because it's a solenoid, it's either open or closed -- no in-between. The linkage on mine broke recently (the furnace is going on 27 years old now), which was a blessing in disguise. I never liked using the thermostat, since when the door was wide open, the coal would sometimes burn hot enough that the firebox door would expand and be harder to open. This wasn't much of a problem when you had the thermostat at a constant temperature, but if you lowered it and then were trying to bring the temp back up later, this would happen. I don't know if this was unsafe or not (no damage or warping anywhere), but I didn't like it when it happened anyway. Now I can always keep this air inlet closed, regardless of what the thermostat is doing, or manually crack it for more air if needed.
As for burning in it, I found you really need to load the firebox all the way to the top of the lowest row of firebrick -- about 6 inches thick. Any less and you get short burn times and a lot of unburned coal. When it comes time to shake the ashes, I open the ashpan door for 5-10 minutes to revive the fire (could be different timing depending on when through the burn you decide to shake). Then I found I need to shake vigorously, sometimes 10-20 shakes back and forth or more, as the grates don't always let down as much ash as I would like. I keep going until there are burning coals in all parts of the ash pan, not just the back (the front burns quicker, and typically has a thinner fire by the time it's time to shake the grates). If I don't do this, the ash builds too thick and I can't get a good burn for long (or the front of the firebox doesn't always catch when I reload).
One of my biggest complaints is in emptying the ashes. The ash pan seems undersized, doesn't hold enough from a single good shaking when needed, and doesn't catch all the ashes. You need to do a lot of shaking, too. In addition, you need to have the firebox door open to reach the shaker grates. It is a messy, hot, and annoying process. From what I've read, none of the other coal furnaces or stoves have this issue.
After shaking, I leave the ash pan door open again for 5 minutes or so to make sure the vigorous shaking doesn't hurt the fire if I had let it go a little too long before shaking. Then, I load the firebox up, completely covering the fire, and close all the drafts (except the drilled holes of course). Takes an hour or two, but it then starts throwing a lot of heat again. Since I normally shake right before bed or before going to work, I don't mind that it cools off a few degrees before the fire really starts going again. With this process, I can typically burn 12-16 hours between shaking the grates. A little less if I crack the draft for those cold days down in the teens or below.
Regarding fresh air, I needed to crack a nearby window for a fresh air supply, or there is a vacuum that builds up inside the house. I was considering running a fresh air inlet to the draft, but considering I may get a stoker stove and just keep this for backup, I decided it wasn't worth the trouble.
My last area for comment is the electricity issue. I don't mind paying the electric bill for the blower motor. A 5.8 Amp motor doesn't use that much juice, especially when it's not on constantly. However, when the power goes out, you have the chance for this thing to overheat. That happened to me once, and within an hour, the fire was hot enough that the air filter melted, filling the house with smoke. Needless to say I thought I was going to burn the house down, but at least I was home to deal with it. Well, the power came on, and I blocked all air inlets to the furnace, and recovered without any more issues (and no permanent damage to the stove, either). It turns out the blower is needed to keep pulling the heat off the heat exchanger, or this situation can occur. Yukon recommends burning half the normal amount of coal in a power outage, but what do you do when you have a deep bed of coal already? How do you keep a 3" bed of coal burning, as well? I don't like that recommendation, as it's just not practical. In the end, I bought a battery backup system. The fuel savings of burning coal in the first year alone paid for the backup system (not to mention the peace of mind!), so I had no concerns with this. I check this backup system regularly to ensure it's working well.
My overall impression of this furnace is that with work/modifications and some learning, it can burn coal well. I am able to keep my 1200 sq. ft. ranch over 70 degrees without trying unless it's well below 25 degrees (which thankfully in NJ doesn't happen too much for long periods of time), and with the drafts closed. On the warmer days (upper 30s and 40s), I'm usually up near 80 degrees and have windows cracked. It's also nice to have the oil backup in case I need to leave for a few days. So far this season I've burned maybe 1-2 gallons of oil when it was really cold and I needed a quick blast of heat once when I let the fire go out (also helped get a good draft when starting the coal fire). I've been burning about 40-50 pounds of nut coal a days.
I would not, however, recommend this furnace, as it doesn't seem to really be designed for coal as it needs some modifications, and for me at least doesn't burn well using the supplied thermostats. In the end, I'm pretty much sold on getting a stoker stove for the living room and relegating this beast to back-up service only, in part due to its age, in part due to the real mess, in part for peace of mind, and in part to be able to get a more controllable burn.