Grandad built a wood burning furnace first for the shop at their place in Powell Butte, and then one for the house. The shop heater was two thick walled 55 gal. drums laid on their side. The lower drum was cut on one end for the door and on the side of the other end for the flue. The second drum was stacked above it with about 1' gap. It's flue-in and flue-out holes aligned, but there was an internal baffle fitted from the rear that ran ~3/4" of the length of the drum. I can recall a couple of times where the heat that thing put out made that area it was in unapproachable.
The second was an all steel thin plate weldment (mostly 3/16") that used the same baffle principle, but in one enclosure. It would heat the whole 2k sqft house and it's 2k sqft basement when really going and from lodge-pole pine and the occasional Juniper it generated only fine ash. Wish that I had a picture of it.
Both drafted very well. Seems to me that something similar should be employed with any wood heating system. Any heat escaping out the chimney beyond that needed to maintain the draft is wasted heat.