To anyone burning coal in a Hoval ZK boiler

[Morith]

I have been trying with moderate success to burn nut coal in my Haval Zk 35/50 boiler. There is no barometric damper in use. My problem is that i get glowing red coals but my oil burner keeps kicking in because the temperature drops below 170 in the boiler. My stack temperature hovers around 150degrees F. What am I doing wrong. Could it be a draft issue. The chimney is 2.5 stories high and draws very well. Everytime I load in more coal the temperature drops. The boiler is capable of 100,000BTU and is slightly oversized for my house. When the outside temperature is above 20degrees I have no problem. The boiler chugs along at 190 degrees with no complaint. Thank you for any suggestions to point me in the right direction.

 

[Hoval]

I have a Hoval that I had to learn how to use. Changes I have made include turning off the oil burner in the winter as I used almost as much oil as I burned without coal. I stopped trying to maintain a high level of temperature and let my boiler do the rest. The second thing that I did was take out the damper in my flue as it would offset the draw of air throuth the coal. Unlike wood you need to have the outside air go up throught the coal not just be there to burn. The third thing I did was to replace the mechanical over heat valve that would send hot water down the drain when it got to hot. All I was doing was heating up my septic system and not heating my house. I used a aquastat I think it is called. This is a relay that would turn on a zone circurlator on my longest zone and put the heat into the house. This stopped the hot water from going down the drain and as I got better with controling the planned burn for the day I stopped over heating my house. I have a 55/50 Zk and that has a damper in front and a damper just prior to the flue in back. I have never used the flue in back just the mechanical on on the front. What do you think about removing the oil burner and replaceing it with a pellet burner eithor a Pellx or Janfire?