So I am a newbie with OWB's. This is my first year using one after 30+ years of heating with wood stoves. I have a lot of 16/18 inch wood stove ready wood in the shed but I have been harvesting standing dead or damaged trees to use in order to preserve the small stuff for maple syrup come spring, had a shortage of firewood last year. Since my unit has a fan which blows up from under the fire and also above the fire, I have been tossing in 36 inch pieces of Cherry and Ash unsplit on the fire then I toss some beech or maple blocks, around 16 to 20 inch on top, everything burns front to back. Poke them into place with a shovel if needed. No complaints at all. Have used little of my small wood so far. I love OWB's. So going forward I will cut and split some 20/24 inch ( splitter is max 24 " ) for the boiler and the evaporator, and any logs under say 8 inches will be cut and stacked at 36/38 length, not split.
This idea of having a fan blow from under the fire bed is available now in maple syrup evaporators, these all come with an insulated door with heavy latches, my neigbour bought one last year, the fire just roars.
Bob