I want to heat and cook in a cabin. I am curious about building a rocket mass heater to meet this need. I have a lot of questions to see whether it's a good idea, and, if so, exactly how to do it.
I know there's a huge amount of info in this forum already, so if you are aware of specific posts that answer my questions, please do point me to them. I'm just having trouble pulling all the infomation I need together into one place.
Situation:
1360-1540 ft³ room volume, depending on how thick the slip-straw walls are. That range represents 12" thick (leaving a smaller volume inside) to 8" thick (larger volume). I'd welcome input on how thick (i.e. how much insulation value) makes sense given how small the footprint is.
Here's data for my climate (how it used to be, anyhow):
| Oct | Nov | Dec | Jan | Feb | Mar | Apr |
|---|
| Daily Average (°C) | 8.5 | 2 | -3.6 | -7.1 | -6.9 | -2 | 5 |
| Daily Maximum (°C) | 13 | 5.5 | -0.5 | -3.3 | -2.6 | 2.5 | 10.2 |
| Daily Minimum (°C) | 4.1 | -1.5 | -6.7 | -10.9 | -11.1 | -6.5 | -0.2 |
| Extreme Minimum (°C) | -6.2 | -19.5 | -27.4 | -31.1 | -31.5 | -32.5 | -15.4 |
I am okay with a cool room in the winter, 14-16°C, as long as there's a warm spot to sit. That's one of the things that attracts me about the mass bench idea - keep me warm rather than the whole room. However, I want the capacity to warm it up to 20°C or so if I have company who likes it warm.
I am working on the land, so I can tend the fire somewhat, but don't want to babysit it all day. I am fine with it getting pretty cold overnight as long as the room doesn't freeze.
The floor is going to be softwood, the walls light straw clay, and the roof steel for now, hopefully switching to thatch within a few years. Support to ground for the mass would be challenging.
I can and will run calculations on the dead load capacity of my floor once I have a clearer idea of the shape of the mass (which affects where the weight lands relative to the floor supports).
Questions
Cross-section of system:
I understand that one can build a variety of cross-sectional area systems, with 6" diameter being the smallest that usually works. I imagine for such a small space it would make sense to keep it as small diameter as possible: 6". Does that make sense? I've heard rumours that you actually need to run quite long fires with some rocket mass heaters to be warm enough, which isn't all that appealing given the repeated feeding required. Would 6" be large enough to allow relatively short/few fires per day?
Stovepipe:
I got some insulated stovepipe for only $25, but it's 7" ID. Is it possible to increase from 6" to 7" at the transition to insulated stovepipe? I think this is considered a very bad idea with conventional woodstoves because the gas expansion results in cooling that may consense water vapour, leading to creosote adhesion, leading to a chimney fire. But I think the exhaust from a rocket stove is already below condensation temperature at this point in the system, so does it matter?
Mass sizing:
I imagine a 6' long bench would be nice so I can sit or lie on it, but I'd rather not make it bigger so it doesn't take up more of the room. I would like to do a stratification chamber design; would a 6' bench be enough internal surface area to absorb enough heat to be efficient? How thick would the walls need to be? This correlates to: how much does it need to weigh to provide enough thermal mass? I need to make sure the floor can take it. It would be nice if the mass is big enough to let me be away for a day or two without the room freezing - is that realistic?
Fire safety:
How can I install the rocket stove and mass so they don't set the floor on fire? My impression is the light straw clay wall should be fine being near or against the mass - is this correct? What about a structural post in the wall buried a few inches into the LSC?
Cooking:
How well does it really work to cook on the barrel? Anyone here using it as their only cookstove in the winter? Are there options that work better than a barrel? Is it possible/practical to build in an oven where the exhaust first enters the bench?
Materials:
I have a powerful ethic of living my life using low-embodied energy, low-tech, local materials. The things I don't love in some rocket mass heaters are: purchased firebrick, wool insulation, foil tape, the barrel, stovepipe, metal heat shielding, perlite/clay mixtures, hardware cloth or hog panel for forming the cob bench... that sort of thing. Perhaps you get the picture. I will use this stuff if I'm convinced it's the best overall option available (compared to much worse performance, or a different kind of stove), but I'd like to push the envelope: how localized can this technology get? How much of it can be made of cob? What options are there for effective homemade insulating firebrick? Are there other chimney options?
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