I wasn't going to bother, but in case someone new to coal burning is reading these threads its I feel its important to give some corrections for future reference.
There are some generalities that are stated as fact that need to be corrected. Individual stove/chimney/house location/lot topography will give the same stove its own 'personality' with respect to draft from one house to another. And where the sweet spot is for the spinner knob and whether an mpd and/or baro gives the best performance will differ from one house to another. We post our draft numbers, stove / stack temps, damper usage (or not) and spinner knob settings as a starting point for new users to begin making their own adjustments and to show through experience there is no one single answer.
To repeat someone else, the internal baffle plate is not even close to an mpd.
The statement that you don't need a baro because of the spinner knob on Mark I contradicts what the Harman manual itself states..."A barometric damper may be installed to prevent excessive or erratic draft."
I burned with a mark iii (with a baro, no mpd) for about 20 years and agree its built like a tank with its 1/4" construction. I still have one sitting in the corner of the basement waiting for the week long power outage that will cause me to swap out the stoker for it. Overall I was very pleased with it and the money saved by heating with coal, but it has its weaknesses with respect to coal burning compared to other units that should be mentioned for 'completeness'.
No secondary air control, no bi metallic air control, and the need to 'scrape' the front & sides of the fire box to thoroughly get rid of the ash there about once per two weeks indicate a less than ideal design. The other weak design feature is the ash pan dimension compared to the grate area. You need to have a flat ash shovel to clean out the sides of the ash pan area about once a week or more. The fact that it has to use a fan to get the most heat off it and into the room should tell you something. Getting the fly ash off the top of that baffle plate is real fun too. And you better do it because if you don't shut down occasionally and wedge a vac hose up there it really hurts the heat transfer from the firebox to the distribution air flow channels. Leaving the fly ash up there in the off season will eventually degrade the baffle plate severely and could cause it to warp.
The service from them was poor in the case where I needed it. On my second Mark III the linkage from the shaker arm to the grates had a poor weld that broke after about two weeks of use. The grates were never jammed in this two week usage so it wasn't misuse, you could see where they missed the weld point. After about a week of daily calls to the dealer and directly to Harman I finally dragged it out of them that their solution was to have me, not the dealer, arrange to have it fixed, pay for it myself and submit the bill to be paid back. I had to find the local welder that would make a house call because i had no interest in wrestling it back up the steps from the basement, load it on the truck and reverse the process after the weld was fixed. I have a network of people with these skills so I had no problem arranging it, but I wonder if a 'regular' user would have the contacts to do this and why wouldn't the dealer or Harman have taken the lead on fixing this two week old stove. It only took about 8 weeks if i remember correctly of constantly calling to get the reimbursement check from them. I have no doubt if I had not badgered them with calls I would never have seen the check, I was less than impressed with how they handled this.