Here are my rmh plans. Minimum footprint, burn box on top, small bench on side. For a 12x16 cabin

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

I'm not an expert on Matt's designs, but I don't think you will have any issues with this build other than the fact that red brick will not play well with combustion zones.

Expect the red clay bricks to rapidly crack and disintegrate after multiple firings.  Unless of course I am mistaken and those are an exotic variety of insulated brick I am unfamiliar with.

Red brick is ok for any other part of the build except for where the primary and secondary burns are taking place, and where the main flow of hot gas hits on it's way out of the combustion chamber.  

Edit.  I missed the part about you lining the firebox with firebrick, so disregard my unnecessary advice.  I too am interested in a "down burn" configuration as I don't want to have to get down on my knees to load a firebox.  Looking forward to your progress.

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.

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.

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.

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

Re: Downburning wood stoves.  I have seen European designs where the fire burns downwards.  They are beautiful to watch in operation.  To achieve this, the operator starts the fire as normal, but with the flue selector set for "bypass".  Once the flue is warmed up, the selector is set to "downdraft" and the fire is then forced to take a downwards path.  

Downward fire needs something forcing it to move downward, like a vertical chimney which creates draft. I cannot imagine that a fire would start in a cold system and move downward.

12 years ago there was much made of the idea that an RMH does not need a chimney; this was the case in Ianto Evans' location on the Oregon coast with a constant prevailing breeze in one direction and the exhaust on the downwind side, but will not be true in most locations in the world.

J-tube combustion cores are indeed reputed to be tricky in 4" sizes, but batch box combustion technology does scale up or down reliably, and moreover gives considerably more heat in the same burning time than a J-tube system of the same size.

If you can't or won't run a vertical chimney through the roof, which gives the best results, I would go through the wall as high as you safely can to increase your chances of success. A long cold exterior chimney can cause cold plugs in certain circumstances and make it impossible to get draft started without going outside and preheating the chimney.

I'm in agreement with Glenn.  While a downdraft is doable, you need a warm vertical chimney to pull a draft strong enough to force the fire to burn downwards.  Hence, the bypass on the stoves I mentioned.  Anything short of that and I think you'll find your living quarters full of wood smoke in record time.

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.