three sides and an open top would create some drafting, and if you had a fan in behind, you'd have good air movement.
feeding some outside air into a wood stove is never a bad idea if you run hot fires in winter. I'd start with 50% of the chimney CSA
I'm not in a place in life to spend 2000$ on a rocket mass heater .
Sean, From what I have read, in the spirit of Ianto's RMH's, they are made mostly out of recycled materials which are picked up for free or minimally priced so one could be made essentially at no charge other than your time. Perhaps the $2000 RMH you speak of was a fancy one made with a stainless barrel, a ceramic fibre core and granite slabs. Mind you, the better quality materials you use, the longer and hotter the stove can burn without premature failure. Just be advised that when you use a second hand product (perhaps without ratings) around something like a very hot fire, you should be cautious to monitor it closely until you are comfortable with its safe performance.
fire brick is expensive i hear
Firebrick (the heavier wood stove liner kind) is about $2.50 per brick in my area. If your just making your firebox out of them and go with a perlite/clay heat riser the initial cost is reasonable for a tight budget (around $50)
but i cant help but wonder if vermiculite and cob would do the job minus being sturdy.
I've made some vermiculite bricks with just enough clay slip to hold them together that I had once installed in part of my burn chamber (for comparison) and I was quite surprised how they held up. Of course they don't take the beating very well near the feed tube like a firebrick would.
another thing i am considering is adding mass around my existing stove.
Adding mass around your woodstove seems quite doable as well.... perhaps something to start with before you remove it and replace it with a RMH. I would make sure to leave a way to properly clean the stove encased in the mass though unless you don't mind hacking away enough to get access to it properly.
i am also thinking that maybe an air intake directly into the stove from outside might help.
The inside/ outside supply air debate has been going on for a long time now.... much has been written here on permies. Personally, I go with using the room air but each situation is different.
The rest of your plan sounds fine to me. I would go with a larger/wider stone chamber rather than trying to replicate duct channels; the rougher walls might impede flow, while a bell chamber dug into the floor would give maximum effectiveness with flow being almost irrelevant.
I fall into the room air side of the makeup air discussion too. Unless you have a hermetically sealed space that needs specific combustion air introduced, you will not have supply issues, and a heated floor will quickly counter any cold drafts. The amount of combustion air needed is not significantly different from the amount of fresh air recommended for inhabited spaces anyway.
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