If I may chime in, I think there is a point at which a "long burn" is long enough. When my stove fires for a 15 degree drop, by the time the temp starts walking back up, that fire is raging! I think that there is a point at which the fire reaches a maximum temp or intensity and it stays that temp for the remainder of the burn. Therefore, I would think that a burn that lasts 15 degrees of water temp swing for 200 gallons is the same amount of clean as a 15 degree swing for 500 gallons. I would argue that a 30 minute burn is just as clean as a 60 minute burn. I think based on the discussions that I've read on here, that we can all agree that the fire box doesn't reach this max until longer than a 5 degree burn, but less than 20. Otherwise, we'd all be shooting for a 30 degree swing or something like that. I'm just swinging by the seat of my pants here, but that's what I was thinking.