Simply because a big log burning like that only burns on the surface, thus produces a less intense heat resulting in lots of burnable gasses escaping.
That guy will have to clean his chimney/stovepipe very often, since those gasses condense once they cool off a bit.
Splitting the wood gives a bigger surface area to burn= more intense heat= cleaner burn, and those gasses get turned into heat instead into smoke.
Small diameter wood, up to 6" I don't split, it has enough surface area to burn well.
My furnace has a blower that keeps it burning hot enough that I can actually stick a piece of rebar in there and make it red hot.
In order to utilize the heat I have a 400 gallon insulated water tank as buffer, then I draw from that all day.
The buffer tank makes the biggest difference. Without that the furnace can only generate as much heat as the central heating system can absorb, or it'll start boiling.
With my buffer tank, I can run my furnace at full throttle for a few hours, and heat the house 24 hours. When my furnace is warmed through and burning there is no visible smoke coming out the chimney, only heat waves. No smell of smoke either.
Did that suffice as an answer, Carl?
This year I took a load of dry wood over to him (2 years seasoned) and he was amazed at how hot his stove got
