Cap wrote:I have the same problem using a Harmon Mark III. I cannot keep a coal fire going for 12 hrs unattended, maybe not 8 hrs. This will be my 4th season burning coal. When it dies, I have at quite a bit of unburnt coal left behind. I believe my problem lies within my flue. The Mark III has a 6" outlet. My flue is 8" x 34' high. My thinking is as the outside temps drop overnight, and the stove cools off a bit, my draft slows down allowing the fire to die. I typically would add a wood log on top of the fire at night in order to maintain a fire till morning. Does anyone believe my theory is correct? I am debating adding a 6" ss liner but at $550, I want to be sure this is the answer.
Lehigh Twp, PA
You're venting a 6" stove into an 8" flue and the area ratio is about 2.2....that is, your 8" flue is 2.2 times the area of your 6" stove exit area and this kills your draft for two reasons: 1) when rising gases "see" this expanded 8" flue area, they expand and slow down and 2) they cool off, both of which reduces draft.
Installing a 6" liner all the way to the chimney top takes care of the area ratio problem but if you have an exterior chimney where say, 3 of the 4 sides are exposed to the outside, then the liner will be sitting in a "cold" flue tile area and draft may also be negatively affected. When I installed my wood stove liner, I put 1/2" insulation on the liner and then shoved it into the flue. This takes care of the area ratio problem and keeps the liner warm, both of which improve draft.