i have a wood boiler that i want to also burn coal in to supplement my wood. my question is this, is trhat ok to do?? i have lined the bottem of the boiler with firebrick and instead of the wood sitting on the botton the the boiler ( and on the brick) i have put in a cast iron grate across the lower part of the boiler so there is a place for the ash to fall and to keep an under-draft going so the coal burns completely, i have just put a couple of fireplace shovelfulls of coal in and i always have some unburned lumps and clinkers in the ash. now is this ok to do or should i just stick with wood and not use coal to bank the fire at night? any advive will be greatfully accepted and followed from those who know more and who have been doing this longer than i have have!!!