Need help fine-tuning in-floor design (cordwood cabin)

Hi all,

Been lurking for a while and got some great information. I need some help...

We have an upcoming rocket mass heater project in a cordwood cabin, 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.

Edward

Edward,
I too am working on an in floor in a small space, but I am doing a green house. I am using 8". I had thought sand/pea grave at first as well. I have reconsidered this due to two issues, first being the natural clay and sand with some straw will help the pipe support weight structurally, and second, the air around the grains of sand rock is not the best thermal mass. When we dug down we found really hard packed clay, so this pile of indigenous clay will be mixed with sand and pebble and packed in to make the floor. Since I am being careful with water by using aquaponics in this green house, I can keep the floor dry. It has stayed dry through several melts and an inch plus of rain through the winter.
I'm good with the floor heating up and radiating. I just won't heat more than I need. Do remember to put clean outs in the bends.
I'm not an expert, but I thought feedback from another novice would be better than no feedback at all.

Hi Beth,

Thanks for the feedback. It's nice to hear some advice/experience from someone new at this too. Also, always nice to hear a different approach.

Edward

Here is a link to an old thread. The in-floor heater in this Yurt is a great design.

https://permies.com/t/5937/rocket-stoves/rocket-mass-floor-heater-finally

Theodore A. : hope your project is still coming along, as we haven't heard from you for a while, I have worked on a couple of builds with experts, and the stoves we built
have worked! Every one who has done both, say that they would never do a in-floor Rocket Stove for an initial build, and then say that they will not live any other way !

I applaud your decision to super insulate your cabin, but I am asking about your plans to insulate your foundation walls, without which you are going to drive a large part of
the heat that you put into your thermal mass to the out of doors, this also adds in drainage issues for your foundation and the need to keep your floor area dry with what
ever heat source you go with !

When you consider the foot print for your in-floor Rocket Stove, have you planed for the excavation below the level of the bottom of the horizontal pipe running inside your
thermal mass ? Your leveled sub-floor,insulation, and1st brick level, and then the layer for the bottom of your Feed Tube and Burn Tunnel !

While there are few shapes in nature that are as strong as a circle, you are going to need something else other than Gravel to encase the stove pipe as it travels horizontally
thru the Thermal Mass, the use of cob is primarily for an efficient heat transfer medium, but it needs to be wrapped tightly around the stove pipe both for complete contact
for heat transmission and to keep the stove pipe from failing, collapsing your only heat source and requiring you to dig up the floor and do a quick rebuild ! Also have you
given thought as to where your clean out / inspection TEE(s) will have to be located and still leave room for built-ins ?

As I re-read this, I find that this may have come acrost as negative ! This was not my intention, and I must guard against this, as I have unwittingly upset other members
of these threads ! In any case lets 'hear' from you, and share some plans so that we might help you !

For the Good of the Craft ! Be safe, keep warm! PYRO Logically Big AL ! As always your comments are welcome !

Theodore A. : If you want to see what is necessary to start a below grade, Rocket Stove Bases foot print, you should go to Martin Seidels build in his Thread
'My 1st RMH build', his build was in a basement and he had problems with his Exhaust gases flow to his chimney but those problems were proved to be
unrelated to the Installation of the Rocket Stove ! Best !

For the good of the Craft! be safe, keep warm, PYRO Logical Big AL

Thanks, Al. I appreciate the advice. I didn't realize this thread was still active, and I missed some of your feedback until now. I built the system, and I'm having some backdraft related issues. I'm going to start a new topic and post some pictures to get reopen the conversation. Thanks again.

Edward