Hi Oliver. This is a great question.
Whereas I generally favor bringing outside air directly inside the stove, the two (or three) different burn modes of the hybrid cookstove masonry heater design will likely require that outside air be brought near to the firebox and ash doors, but outside of the stove. This will allow each of the doors to operate as air controls.
I have been meaning to update this drawing (which was originally published in the design’s development topic):
If you ignore the cooktop as an air intake, the illustration shows the burn configuration in mass heating mode. This is basically a self-feeding batch box with primary air coming in through the door and secondary air coming through the grate.
What the illustration does not show is what happens when you reverse the flow using dampers upstream in the design to move into cooking mode where the flames are traveling upward towards the cooktop. In this configuration, the ash door and grate administer the primary air and the door administers the secondary air.
This dual function is one of the main innovations of this hybrid design.
I was extremely impressed with the quality and apparent cleanliness of the burn in both of these configurations in the prototype that we built at the MHA annual meeting.
So far in its design development, this would require that outside air come up through the floor right outside and at the base of the stove, likely in the tile work of the required hearth extension.
There is a piece of hardware that was designed specifically for this purpose that I don’t believe is currently available:
It was designed to cap and regulate a 4” outside air feed in the floor in front of a stove.
I hope this provides some insight.
Would be really neat to see progress on your build. If you feel the calling, start a new thread with progress reports! Will try to cheer on and provide info. in the process!