Yep, That is the rear ash clean out in the picture. This area will be completely covered in firebrick. I think that the taller the chimney the better. The amount of air going into the primary and secondary however is much more important then a drafty chimney, My thoughts anyways could be wrong! I think the boiler in the pics has a 4 or 6 foot chimney, so its not crazy. There will be ways to control how much air goes where.