I like break the ash discussion into two parts: Combustion ash and fly ash
You are going to get a lot of combustion ash this is just due to the nature of the
wood and inorganic materials that cannot be combusted. Stated differently, you
cannot eliminate ash, you can takes steps to reduce combustion ash and fly ash buildup. I like to clean my combustion ash every time I burn the
rocket mass heater. This helps reduce combustion ash build up and also reduces the change of breaking the combustion ash into smaller fly ash.
Fly ash is smaller combustion ash that is light
enough to be carried away with the draw of the
rocket mass heater. You will need clean out ports in your design. The preferred location is closest to the combustion chimney and adjoining horizontal ducting. In my build I have a fly ash collection pit. This allows the fly ash to specifically collect in a pit before entering the horizontal ducting of the thermal bench. The clean out port is within arms reach. Depending on how much you burn you may need to clean this several times a season or only once a season.