I'n the following link is a sketch of my in floor rocket mass heater with air intake located in the composting toilet unit and the exhaust running beneath adding heat aswell to aid in composting. Please post suggestions so I can refine this design.
A few things come to mind. I hope you are trying to heat the room as well (because of course you will). I am not sure how long the exhaust pipe is under the floor... it looks short as 20feet is pretty normal (some are longer)... this may be that you have just shrunk the picture to get everything in. I would make sure that the burn tunnel is insulated... perhaps with some mass inside the insulation as you want to keep that area as hot as possible for good burning. The angled feed will keep you having to pay more attention to the fire unless you add a trough to hold somewhat longer wood. A RMH is expected to burn for a least a few hours at a time. So you may want to add some mass to your barrel so the room doesn't get too hot before your burn is finished. No need to worry about that at first as mass can be added after the fact if needed to smooth out the temperature cycle... just make sure your feed leaves enough room to add that mass later between it and the barrel.
I think the idea is good, I have never tried a long air intake like that so you are the test case... but it has been done in the floor in more than one place and seems to work. There are even pictures posted in one of the threads in this forum... might be a year or so old.
this is just a conceptual drawing where i have yet to specify the length of "thermal mass exhaust". i agree that the barrel needs coverage with thermal mass in the design.maybe a nice removable bricked in pattern? a big concern of mine though is cleaning and inspection of the system, this is why i want the barrel to be open able for routine inspections. The space i am looking to heat with this system is a Small house about 240 sq ft. with a loft sleeping area. here is the house i am basing my design on.
Ok, so 15 feet or so. It could have a bend in it to make it longer and heat more area or it could be split in two runs. The ones with split runs I have seen do not join back together but exhaust in two places. As far as removable mass goes, this is what I did:
They are held on by wire:
also this brick configuration is EXACTLY what i meant! thank you for the great pictures!
my goal is to have always a positive pressure within the building and a negative within the composting toilet unit.
How? Normally a fire breather creates negative pressure... enough openings would create mostly neutral pressure. Unless you use fans, there is negative ... less negative and more negative. Maybe I'm missing something. Its the right pressure gradient though.
the idea was to create a controlled situation where the fire can pull it's air from and having a negative air pressure within a composting toilet is a big bonus! no stinky air entering the living space! In addition aiding to the decomposition of waste.