This kind of pissed me off.
I have found out, that we have this thing above our heater. Heat spreads using the AREA that is in contact with air. This is why your GPU has metal fins mere mm spread from each other.
I am seriously thinking, if I should just take a cooler from 2 GPUs, and weld it to this device in order to make the heat get to the room faster.
Would it make a noticable difference?
@roboneko
FURNACE!!! I have finally remembered the correct english word. 
But yeah. It is a simple setup, where there is a wood furnace, and this device is placed between the furnace and the chimney to extract more heat out of it.
I get, that the smoke inside isn't clean, but that shouldn't matter, if I connect the heat sinks and fans on the outside of this device right? And since it is still being heated from the inside, the true factor making it more efficient should be the ability to extract the heat out of the exchanger.
is this one of those traditional metal wood stoves that's freestanding or what?