We have an upcoming rocket mass heater project, and I have a few lingering questions that I would love some feedback on. First off, I've read the Evans/Jackson Rocket Mass Heaters book, and I feel like I have a decent grasp of the concepts covered. I'll describe the project briefly and list some of my concerns. The structure we're designing is 12x12, with a net interior square footage of 100 sq. ft. It's being built in upstate NY near the Berkshire region of MA, so we'll need some decent heat. We are building a stem wall that was originally designed to be 12" tall. The layers of the floor will be contained within that stem wall and will leave the floor being about 2 inches from the top of the stem wall (10 inches of floor). The primary in-fill for the floor is going to be gravel. I intend to route the pipe from the rocket stove through the gravel. The layer on top of that will be the earthen floor, which will serve as the thermal battery. We'll have the top of the heat riser pretty close to the top of the barrel, as we intend to use it for some cooking.
-Which diameter of pipe? I feel like I understand the benefits of 6" vs 8", but it seems like it will be difficult to squeeze enough piping for heat extraction with an 8" system under a 10x10 floor. If I switch to a 6" system, how will that change the size of the barrels needed for the feed barrel and heat riser? Will a 6" system generate enough heat for upstate NY?
-Depth of floor...As I mentioned above, the floor will be around 10" thick, does this in any way still leave enough room for a 6 or 8 inch pipe without promoting overheating through the floor system. Don't want to end up with a floor too hot to stand on. The depth of the floor is pretty flexible at this point. We can just extend the height of the stem well to allow for more floor depth. How deep should we have this pipe under the earthen floor? Will I be leaching too much heat to the earth beneath the pipe and gravel to see any real benefits on the floor level?
Path of pipe... I originally designed it to just weave back and forth with elbow connectors, but I was a little concerned that the path I had in mind would offer too much resistance for the heat flow. I recently looked at the design on page 26 of RMH for the daybed, and it encouraged me a little bit to just keep that pathway in the design. Anybody have any alternative suggestions? Split it up into two more direct paths out of the structure? How to cram all of that pipe in for proper heat extraction, etc? As I mentioned above, we'll be using a lot of that heat on the front end of the system with the intention of cooking. Can we cheat the length a little bit?
I know it seems like a stretch to put a RMH in a structure so small, but we're so drawn to the efficiency/future wood savings of this type of heating system.
Okay, that's all for now. Thanks for reading, and I look forward to your ideas.
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