We have been heating for ten years with a US Stove 1557, and have been fairly happy with it. Three years ago we added a Hearthstone Heritage in the open LR/DR, and love it. The Heritage is used for primary heat when the temps outside are above about 30F, and the two unit are both used when temps outside are below 0F or the wind is really blowing. There's a modern LP furnace for backup, and the LP and wood are separately ducted systems. I'm considering upgrading the HotBlast -- it is about 15 years old (it came with the house), though still runs fine, but it is hungry and dirty. The only reason I'd like to upgrade is to burn less wood and burn it more cleanly. We like that it can burn any wood of any size that fits in the box, and I know that this will not be possible in a more efficient furnace. The burn times are good enough for us -- a load at 11pm will usually leave a good bed of coals at 7am. The upgrade is constrained by the fact that the air ducting will not handle much more CFMs than the HotBlast pushes (1100cfm). The main trunk is only 8 x 14. This seems to rule out PSG, Drolet, and Kuuma. I'm not willing to reduct the house. The house is two story, 3000 sq feet, basement is partially heated, insulation is decent, house is 100 years old and not sealed overly well. The HotBlast/Hearthstone combination has been working well, temperature-wise. The wood furnace is installed in what used to be an attached garage (but of course we don't park vehicles in it; we have another detached garage for flammable equipment). I'm considering the Ashley AF700 or AF1440E/AF1500E (I don't know how these last two are different from each other). For both, required clearances, ducting, CFM and BTU output, etc are similar to the HotBlast, so should simply plug right in to our current setup. I'm most concerned about burn time compared to what we are used to. The AF700 has 3.6 cubic feet firebox; the AF1500E has 5.2 but is 10% less efficient on the EPA tests. The HotBlast has about 6.5 cubic feet firebox. I'm under the impression that firebox capacity is the primary determinant of burn time, but I don't know how changing to a unit with secondary combustion will affect this. Any advice is welcome.