Yes, I was a bit cryptic when explaining my setup. A little more detail. The horizontal flue runs from the drum straight for 9m with no bends. Then a 90 degree bend horizontally and immediately another 90 degree bend vertically where the 3 meter chimney follows. The area of the gap between the drum and the horizontal flue is about 1.5 times the area of the flue pipes (8"). So no bottleneck there. In fact, all the dimentions adhere to the specifications as set out in the RMH "bible" pdf
I started burning the stove again and again and it looks as if you were right allen, the mass needs to dry out. The longer I burn the better the draft gets as the bench starts heating up. Yes it is a large bench. It is 9 meters by 0.7 meters by 0.42 meters. The flue as right at the bottom of the bench so it is covered on top with 220mm (8.6") of mass. The sides are covered by about 250mm a side of mass. I went overboard I think.
I did some temperature measurements. I have sensors touching the top of the bench in 3 places. Starting the fire the bench temp was closest to the heater 15 degrees C. After around 3 hours it went up to 22 degrees. It then slowly cools down through the night after the burn finished and reaches about 15 degrees again round about 6am. I will do some more tests and post some measurements later. The temperature last night in the clivia area (shadenet covered with greenhouse plastic, very poorly insolated) went down to 2.88 degrees with the outside temperature drommping to -1.5 degrees. I only need the Clivias to remain above 0 degrees or slightly below, not colder. So so far it seems to do the job.
For those interesed I also added antracite to the feeding hole behind the last logs. The antracite all burnd out during the night, might have helped lowing the cooling of the mass during the night as well but I will have to do more tests and measurements.
I still have a problem or maybe it is just the nature of the beast. The pdf states that an 8 inch system can burn logs up to the size of one's knee. I have smaal knees
and still I can not burn logs that size in my heater. For some reason the logs dont burn fast enough or maybe too fast forming "chimneys" between the logs followed by flames running up the logs instead of burning sideways into the burntunnel. Using smaller logs is fine (or better). The wood I am burning is wattle, very hard and difficult to chop. Takes a lot of effort to get it going as well.