J-Stove Rocket Mass Heater

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Core Oven: refractory board rated to 3,000 degrees.

I thought I’d make my own super-efficient Rocket Mass-Heater for the ranch, to replace the dangerous old cast iron wood stove. A Rocket Mass-Heater burns about 80% less wood than the old stove, can run on pellets as well, produces no smoke, few gasses, and will keep the house warm all night with no fire burning. It provides a radiant heat source via a stainless steel bell, as well as a large radiant mass that warms to a few hundred degrees which then radiates heat for 8 to 10 hours after the fire has burned. It can heat the house for 12 to 24 hours for one hour of burning- depending on how well my mass structure absorbs and retains heat. I will build that up at the ranch, using aircrete; concrete blended with super foamed soapy water. This reduces the weight of the concrete mass by 60-75 percent, and allows the heat to permeate the mass.

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This is the Walker J-Stove design. I bought the layout plan for $20 online. Brilliant.

The interwebs are awash with bad Rocket Stove designs. Many cores are created with bare steel using old gas canisters. At 1500 to 2000 degrees in an oxygen depleted environment, steel undergoes a process called spalling. Essentially it rusts without oxygen, or more precisely it is the effect of reshuffling the iron molecules wherein they lose covalent bonds and layer like sheaves of paper. Refractory material is the only media that can survive the temperatures in the core and stack. I found the best understanding of the forces at work were from Masonry Stove builders, in particular Walker Design, who offers a J-Rocket core design to keep us diy dinks from burning down the ranch house. My design is a hybrid, as the stove will stand alone, as will the mass-heater. This is borrowed from another solid innovator, The Honey-Do Carpenter. I’ll be using his specs to create the concrete foaming gun, and modifying from his prototype of a light weight mass heater separate from the rocket stove.

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I added this air-flow channel, bringing fresh air to the riser to encourage the venturi and oxygenate the superheated gasses for a complete burn.

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The air splits into three channels, that combine into one, with the three channels still delineated at the top. This opens directly into the stack.

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I found this round refractory tubing rated to 1500 degrees, perfect for a venturi stack, or chimney. The Walker design uses the same media as the core, so that stack is square, which limits the gyre.

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I wrapped the stack in heavy perf aluminum, and tied it with stainless steel wire. 

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Now I wait for the fireplace brick splits order to arrive, to line the inside front of the firebox. This protects the delicate refractory media from the wood fuel.

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The fire brick arrives, and I run a hot burn to temper / shrink all of the media, prior to spackle with high-temp mortar.

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As the fire climbs to temperature, there is an initial column of smoke.

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In a few minutes we are at temperature, and the smoke is gone. All wood is reduced to gas, and even the gasses are burned. Her core temperature is 1,500 to 2,000 degrees (hot enough to melt bronze and steel). I can hold my hand against the walls, they are warm to hot.

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Operating at full burn. 

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Only the bottom end of the wood burns. These board-remains were 4 feet long, and slowly digest into the jet of flame. The soft roar of the air to flame is why this is called a Rocket Stove.

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The remaining cinders are the remains of the last few cooler minutes of burning, as the full incinerating force drops.

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This is after the next step; encasing the delicate refractory core in a heat-safe and tough shell.

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The refractory core is bedded in rock-wool, a fireproof spun silica insulation that allows no air movement. The outer shell is hardy backer board with a tough outer shell, ready for tile. I used an industrial cement and seam-webbing to join the board. 

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20 feet of angle iron, and a few steel scraps, welded into a sturdy frame. 

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The stainless steel 15 gallon drum goes over the chimney column. This acts as a heat radiator. 

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I cut this perfect circle to sleeve over the chimney. Next I will cut a small hole on the outside for a stove pipe connection, and another to fit to the mass heater (another build for another day). 

Lots more to do: support the stainless drum; put on low legs; create gravity-assist wood feeder of wide square-stock tubing that is removable; resolve the stove piping; tile it; maybe make a pellet feeder. Then design up the mass heater for assembly in Montana.

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