After using an exterior wood furnace for two years, we've discovered that they are smoky, useless during power outages, and run through lots of wood.
Outdoor Wood Furnace
Legend Outdoor Wood Furnace Manual, PDF, Stove
Providing Outdoor Combustion Air for a Wood Stove - GreenBuildingAdvisor
Choosing an efficient wood stove
PROS & CONS of an Outdoor Wood Boiler! EP 110
Air: Outdoor Wood Boilers Wisconsin Department of Health Services
Providing Outdoor Combustion Air for a Wood Stove - GreenBuildingAdvisor
Smallest wood stoves
Disadvantages of exterior wood furnaces