after around 5 to 7 min the fire should be big enough and already catching on to the birch to put more wood on; put now smaller pieces from light wood in the oven - all in the same direction, one end to the back of the oven, the other towards the hatch - to create a small pile of wood around and a layer on top of the fire and wait till the fire catches up the wood - close the upper hatch in between to not fill the room with smoke
Careful now: When you open the upper hatch and the bottom hatch is not tightly closed, then it can happen that an explosive flame comes out of the oven. Make sure you close the bottom hatch, especially by a stronger flame, and open the upper hatch SLOWLY.
after a short time you will hear the fire burning pretty loud (the heat is that strong that it sucks the air from below pretty fast)
The heat is not pumped to the mill building but the oven is on and the temperature indicator is higher than 62°C at the oven for more than 10min
This is quite simple but it is a bug of the oven/heating system and you have to restart the oven by shutting both switches on the top part of the oven (labeled with 0 and I for on and off) off and on again.
If that is the bug you can see that also by: Next to the pipe is the lowest green light on but not the second lowest (4 lights: 2 yellow (or off), 2 green(or off)) - it only produces warm water for the mill, but not for the heating.