I've burned Kimmel for the past few years and haven't had an issue. Just this week I purchased a ton of Kimmel nut and I can't get any heat out of it!! I'm assuming it's the coal quality, but the ash build up isn't bad...?? Today it's -5 degrees out and the draft from the baro could suck a plastic bag through the chimney...plenty of draft, but the fire is low...uGG
Confirm for me...is it the coal quality??
Perhaps you have clinkers/ash restricting the airflow through the coal bed?
Edit: It is possible that it is the coal, but I would eliminate fly-ash build-up in the chimney (check first elbow or tee after stove), and see what stove temperature you are actually getting. I recently noticed that my EFM was acting "lazy" and that the baro was wide open...there was about 3-4" of flyash in the pipe between the boiler and the baro.