Just to clarify, you can create an air intake that comes through the wall/floor and provides an air source near the wood feed, if you want to minimize any drafts in the house, my previous comment was more for having an intake that feeds right into the burn tunnel so it might not be a clear reply.
Ianto and Leslie's book Rocket Mass Heaters mentions an air intake on page 68, suggesting a 4" intake with mesh covers on both ends to keep out rodents would be plenty for providing air intake but it's not meant to be going into the RMH, just near it for that fresh air, from how I read it.
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