Question for you folks, friend of mine came to me with a question that has me stumped, he has a wood / coal boiler made by Harmon. Burns fine with wood, He has been burning since 01, but he has been trying to burn coal with no luck. Burning hard, nut coal, wood fire first, good red coal bed then adding coal little at a time until it’s burning good, builds the coal up layer at a time, coal burning red & even white on the bottom, but as soon as he shuts the door the fire will die in approx 2-3 hours. If he catches it and gives it air still cannot get the fire burning again. I checked w/him and the unit does have shaker grates. Sounded like he was doing everything right, but I’m out of ideas. Any help?