Hi Yoshi, Welcome to the Permies RMH Forums!
Your question is a valid one, but not anything to be concerned with when your dealing with a
rocket mass heater that produces much cleaner exhaust than most conventional woodstoves do.
Cresote can be a real problem with most woodstoves especially since the exhaust still often has unburnt gasses left and tends to collect in any area that is cool and/or at bends/joints in the piping leading to potential chimney fires.
With a properly functioning RMH, the exhaust will be clear and all you
should see are heat shimmers out the chimney stack. As long as there is
enough heat left in the final vertical chimney to mantain a draft, it will pull the clean exhaust through the pipes like a train or a siphon.
To achieve this, the
wood is burned quickly and at a very high temperatures vs allowing the wood to slowly smoulder at lower temperatures.
Hopefully this helps put your mind at ease and make you want to build one yourself.
If so, lots of people here to help out.
PS. We love pictures.