Brian Thorp

Actually, on my garage build I forgot to mention I used a heat gun on the clay flue section in the second photo. It got the draw going pretty well in lieu of a bypass ... I have since taken the top few rows off and laid fire brick for the roof of the fire box to increase the space under the cook surface. I will eventually redo this First Build with fire brick and clay mortar instead of refractory mortar which has cracked a bit already, but I like the bottom part which is like an underneath bell.

Mark Brunnr wrote:Just to confirm, the wood feed is at the top and when lit the expectation is that the hot gases will draft downward first, then work through the twists and turns to reach the mass and exhaust? Have you tried a dry stack version already, to confirm this will work? It seems incredibly unlikely that it would. But if you start with the wood further down, and have the heat riser come up to the griddle at the top, then the gases could go back down from there. I can't recall ever seeing a design where the exhaust didn't go up at least a little first, release some heat, and then fall back down.

Thanks Mark, I will definitely dry stack first. I honestly don't know how it will perform. I may not make a bench, as the thermal mass of the unit itself will keep the cabin warm for hours. I may pour a radiant floor later and heat a tank of water from the RMH for fun.

In my original intuitive design I envisioned a 6" chimney through the wall at either floor or waist level and ending above the roof line, thinking like a wood stove owner. Then I read the Permies post, "rocket mass heater - exaust can be simpler than chimney"  - 12 yrs ago - about trying a through the wall exhaust first, and if the temperature was under 90 F then go vertical. For the space I could go 4" but it seems they never work well according to Wheaton. I imagine I would have to scale down everything proportionately.

"12 Reasons to put a chimney on a rocket stove:
Draft / Physical:
1) If your exhaust is coming out at 90 degrees Fahrenheit or hotter, a vertical exhaust will likely aid draft.  (This is the rule of thumb from Dale and the Masonry Heaters folks.) "

etc

Glenn Herbert wrote:Your garage heater follows the basics of Matt's design and works. You mention the draft is weak when cold - as the chimney warms up I would expect it to increase.

Your cabin design is different as you secondary burn chamber is smaller and shorter and the flames will be going down immediately. Your garage heater firebox is similar to the sizing for a 5" batch box, while your cabin design fits a bit less than a 4 1/2" system. 6" flue is way oversized for this, and you would have slow flow and even more cooling in the exhaust. Batch boxes depend on a chimney for good draw, and you are planning a system that will have no final chimney AND no internal riser "chimney". What do you expect to be the driver for draft in the desired direction in this plan?

I do think a 4" or so batch box is well sized for a tiny cabin as you describe, but I think it will not function without a chimney, and preferably one that goes up through the roof. If you do that, I think you will have a decent reliable heater.

Thanks Glenn, I have tried to keep the area of the holes in layers 3 and 5 and the tunnel in layer 4 to a consistent 6 1/4" x 4 1/2" = 28 sq in, the same as the area of a 6" exhaust. The burn box is 13 1/4" long x 6 1/4" wide x 9" high. The vertical opening a the back of the burn box is 9" high and 3" wide. So yes I will do a dry run and yes I agree I might have to use a vertical chimney. I am assuming that fire follows the air supply and does not necessarily need a vertical part. I was not aware that batch boxes need a chimney, although my above burn box dimensions are not that big, are they?

I am open to a 4" system because the space is tiny and is insulated on all sides with 2" (recycled) foam.

Hi folks, I've had such success with the Walker design in my garage that I designed this minimal footprint RMH for a small cabin. I wonder, since I haven't built it yet, what you think of the downwards design with the burn box at the top, especially if I add a 7 ft bench and then vent it through the cabin wall.

In the photos of my garage unit you will see how I made the final path around the base before coming up to the vertical chimney. In the cabin design in the PDF you will see it flipped on it's left side to use up less room. I intend to line the fire box with fire brick.

Just a comment about my garage build. The initial draw was weak when cold. I haven't made a bypass yet, but it's possible to put one at the same location as the Walker. And the Pyrex bowls are working out well so far. I install them with fiberglass rope and refractory glue so they have room to expand if necessary.

With this new cabin build I don't see an option for installing a bypass, so I'll just build it and see what happens! I won't be forcing it through 20 ft of chimney so I hope it will pump heat. If it smokes back maybe I could install a small fan at the exhaust at the end of the bench where it's coolest - it's an off grid cabin so I have to use 12v solar power.

Thanks

Brian

Thanks for the helpful comments.  My experiment will be to heat my 3" (insulated with recycled styrofoam and vapor barrier) 12x16 cabin with different kinds of  thermal mass. I am seeing more potential with filling the crawl space with rocks and ducting and connecting a rocket stove to it. Because my cabin is between 1' and 3' above grade I have realized i have to close up and insulate underneath and around the periphery if I want to have a radiant floor above, which would be half a yard or 32 bags of concrete to make a 1" floor. And if that crawl space was filled with thermal mass waiting to be heated by fire during the day it would accumulate enough heat to get through the night with the assistance of the floor which could be heated by different methods. My solar panel dump would keep the floor above freezing while I was away and then I would be able to switch it over to a copper coil on a rocket stove when there.

I'm in zone 4b

Thanks for all the replies. The cabin is well insulated, and I have a charge controller and a battery, but I agree I should try a 300w element.

Hi, I am wondering how well this will work:

I thought I could use my solar panel to heat a drum full of water. I would put a hole near the bottom for the element and another at the top for a pressure release valve. My cabin is off grid and fully insulated with thermal windows, so a small, constant supply of heat might do the trick. I have all the components to set this up for the winter, and I like the passive approach - no pump.

I am in Ottawa. also looking for fire clay. This area has lots of clay, but it's wet. Do I use it wet?