It seems that they evolved into thinking that only forced air ( with fan) can provide a clean combustion and good heat exchanges.
Yes and no. Read it a few more times While they do have some designs with fans, they also have some no-fan designs they claim work well. The first thing to realize is that document is quite old... That design as it shows is not used... there have been changes made. There are some different ways of doing similar things. Aprovecho has decided that quick heat and quick transfer is the way to go. If it is too hot, design to burn less fuel at a time. This may work ok in the climates they work with.... mostly warm but need a bit of heat for cold evenings/mornings... mass heaters don't work well for that. The rocket heater shown there has a riser that is effectively not insulated... the new ones are. This alone is a big part of the not needing a fan to breath. The other concern of not enough heat transfer to the mass has been rectified by a longer heat transfer pipe. Those are the obvious things to me. The rocket mass heater as used today looks the same, but has been improved. Some of the pipe dimensions (CSA) seem to be odd too. They gave up on it before it was fully developed because it didn't work in their case.
In Europe where they have used mass heaters for ages.... hundreds of years, it was found that the farther north they are built the more mass is needed.... farther south less mass is used to deal with outside temperature variability. Something to remember then is to build for your situation.... even in Europe they use steel stoves for work shops where heat is only needed for a few hours during the day rather than 24/7.
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