Hooray for the learning curve. This iteration of a mass heater weighs in at just over 100lbs, and took a day to create.
Rocket Stove Stress Test
When last we saw the rocket stove it had gone through core/chimney test firing, then had been wrapped with rockwool inside a hard backer-board shell trimmed with steel. It awaited the stainless steel drum.
Here the drum is in place fitted over the inner chimney riser, with two vents of stainless steel pipe securely welded near the bottom of the drum. The drum radiates heat to the room, and vents the comparatively cooler air near its base, creating draw on the chimney by dissipating heat quickly. The vent pipes are fitted with double wall stovepipe vent, increasing from 3 inches at the drum to 4 inch vent. This rear vent is the main riser to the house chimney, and the other will feed into and through a mass form (to be built in-situ at the ranch), this mass-venting line will link back into the main riser, ensuring a continuous hot draw out the house chimney, as well as a strong draw through the mass.
Running with no visible emission or smell of smoke for 40 minutes. I need to verify that the rockwool liner survives a high heat and keeps things air tight, that the outer drywall board remains stable, and that the metal frame stays cool. The stainless steel drum begins to turn copper color with heat, as it should, but the box is cool to the touch. Time to test to possible failure.
I pack the firebox with 1inch diameter pine and 3 inch branches, and the chamber, already over 1000F, devours the wood straight into a white light. Temperature at the chimney, with o2 blending into the gas reburn venturi-vortex, jumped to over 1500F or more- and suddenly a putrid heavy black smoke churned from both vents. It was similar to foundry fails, when molten metal is poured into a mold that hadn’t burned out completely, and the metal annihilates everything not burned out and gasses out the uncured mold- in a vertical jet of putrid flame. This wasn’t anywhere nearly as dramatic, yet I recognized the smell of things coming apart in high heat. The black chem-smoke would dissipate, and I would repack the box, and the black would bloom again. After 20 minutes the worst was over, and I pulled out the remaining big branch, and replaced it with a 1×2 x two foot run of oak. This hardwood will burn hotter, and burn very clean; soon things were running back at “zero emission” again.
I have already ordered ceramic fiber board rated to 3,000F, same as the unscathed core-box, to rebuild the chimney. I wanted to try the round chimney, even though it’s temp rating was only 1200-1500F as it allows a stronger spinning venturi-effect to cleanly re-burn all the gasses. Looks like I’ll be going with the square version made from fiber board. I’ll post how the retest goes soon.
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.
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.
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.