Another thing to consider. Is the distribution blower working properly? While it sounds like you are having an overdraft you are still getting a 550 degree stove temp on low burn. At this temp you should feel a strong push of hot air blowing out the top of the stove. I have a Harman 44 Magnum stoker and if I get the stove temp to 550 it feels like I am standing in front of a huge hair dryer if I turn the distribution blower on high. Remember, on Harman stokers you aren't getting a lot of radiant heat directly from the stove you need the distribution blower to push air over the heat exchanger inside the stove to disperse the heat. It also takes time to get the feed rate setting just right for your situation. When I first got my stove I was also pushing a lot of burning coal out of the burn pot because they set everything at the factory to the highest levels to get max btus. It took me a few weeks before I finally got everything adjusted right. Now I have a stove temp (measured on the side panel with a magnetic therm) of 250 on low burn to 750 on high burn. I can adjust the distribution blower speed on my stove so if I don't need the heat I can put it on low or crank it up when I want the heat distributed into the room. FYI - To keep a stove temp of 550 I would burn around 60 lbs / 24 hour. I have a 2400 sq ft house (2 floors with open floor plan) and my stove is more than enough to heat the entire house, even when it gets down to 0 like it has recently.